passage of the pedicle. Muscular impressions stronger than in Lingula.

L. Cambrian to Ordovician.

Obolella.-Shell built up like that of Lingula, but with phosphate of lime preponderating over the horny layers. Form approaching circular, nearly equivalve; valves only slightly convex, and concentrically marked. The beak of the ventral valve is furrowed on the inner side by a groove for the pedicle. Differs from Obolus only in form and position of muscular impressions, those near the centre of the valve in Obolella being widest at the end away from the beak, while in Obolus this end is narrowest.

Both genera have the same range, Cambrian and Ordovician. Discina (fig. 67).-Shell minutely punctate, composed mostly of horny matter. Inequivalve. Form circular, smooth or concentrically marked. Ventral valve flat or slightly conical, with beak almost central; a foramen occurs close against the

Fig. 67.-Discina Forbesii (Wenlock
Beds).

Fig. 68.-Crania parisiensis (Senonian). Interior of ventral valve.

beak in adult forms, and from it a furrow sometimes runs externally towards the margin. Dorsal valve conical, with an excentric beak. The forms with a furrow have been called Orbiculoidea, and those with a ventral median septum Discinisca, leaving Discina only for Recent species. In its usual wider sense, Discina is Cambrian to Recent.

Crania (fig. 68).-Shell showing punctation on inner surface only, the tubules breaking up into a number of much more minute ones as they near the outer surface. Calcareous and fairly thick; sub-rectangular to circular; surface smooth, or ribbed with ridges radiating from the beaks. Ventral valve conical and attached by the actual shell-substance of the beak, which is commonly nearly central. Dorsal valve conical, also with nearly central beak. Both valves have well developed muscular impressions and a characteristic broad flat border marked by granulations. The ventral valve is naturally often found adherent to other fossils, without the dorsal valve.

Ordovician to Recent.

21

In contrast with those of the Brachiopoda, the bivalve shells of these animals have typically equal valves. Moreover, hold the shell so that one valve faces the observer and the umbos form the highest point; a vertical plane passing through the umbos, and perpendicular to the plane of junction of the valves, will divide the shell into two unequal parts. Hence the shells are

Fig. 69.-Sinupalliate Lamellibranch (Cytherea incrassata, Oligocene); left valve. a, Impressions of the adductor muscles. a.l., Anterior lateral tooth. e.., Groove for the external ligament. h, Hinge, with three diverging cardinal teeth; the middle one is divided by a groove. 1, Lunule. p, Pallial line; the infold or sinus indicates the position of the retractor muscle of the siphons. u, Umbo.

said to be equivalve but inequilateral. The longer part is in almost all cases the posterior.

One or other valve may become smaller; but in these inequivalve genera the inequilateral character will probably betray itself (fig. 72). Similarly some genera have shells that are practically equilateral; but a slight difference of the hinge-line on either side of the umbo (fig. 91), or the posterior position of the single internal muscular impression, will often serve as a guide.

The classification here adopted seems suited to those who unfortunately have to deal with empty shells rather than with living molluscs; but it must not be regarded in all cases as an expression of the nearest zoological alliances. The separation

of Leda and Nucula is a marked illustration of this. (See the interesting discussion by F. Bernard, Paléontologie, pp. 541-543.) Terms used:

Anterior Border.—The end of the shell where the mouth and foot were situated.

Posterior Border.-The end of the shell where the cloacal aperture and, in siphonate forms, the siphons, were situated.

Umbo.-The beak or apex of either valve. This in the majority of forms is directed forwards-i.e., towards the anterior end. Sometimes bent round or even pointing posteriorly.

Hinge-line or "Hinge-border."-The line along which movement takes place when the valves open.

Ventral Border. That opposite to the hinge-line.

Right and Left Valves.-The shell is held resting upon its ventral border, and the anterior border of the shell is directed away from the observer. The "right" valve is then to his right, the "left" valve to his left.

Ligament. The "external ligament" (by which the valves would be opened unless held closed by the muscles within) is placed, in the main, posteriorly to the umbos, and sometimes. leaves an impression above this part of the hinge-line. The "internal ligament," or "cartilage," lies within, below the hingeline, and is set in ligamental grooves or pits (figs. 84 and 93), which are seen near the hinge when the animal matter has disappeared. It may be remembered that the internal ligament becomes compressed when the valves close, and that its expansion causes them to open directly the muscular pull is released. Sometimes only ore of the ligaments is present.

Area, or Escutcheon.-A generally elongated oval area seen behind the umbos in some genera, when the valves are united, and running some way along the hinge-line.

Lunule.-An oval area seen, when the valves are united, in front of the umbos.

Gaping. When the valves are closed, and yet leave an opening at one or both ends, the shell is said to be gaping.

The following structures must be noticed on the interior of the valves:

Cardinal Teeth.-One or more processes, fitting into sockets in the opposing valve, and arising near the centre of the hingeline. The teeth thus alternate in opposite valves. The true cardinal teeth arise below the umbo, but may extend back obliquely from it, so as to stimulate lateral teeth.

Lateral Teeth. Similar processes commonly ridge-like, towards the anterior or posterior end of the hinge (figs. 69 and 73).

Muscular Impressions.-These are shallow, fairly circular, or

pear-shaped pits representing the surfaces of attachment of the adductor muscles, or muscles used in keeping the shell closed. Sometimes only one (the posterior), sometimes two, are present in each valve. When the two impressions are fairly equal in area, the shell is that of a Homomyarian; when the anterior impression is smaller, it is that of a Heteromyarian. The animals with only one adductor muscle are said to be Monomyarian.

Pallial Line. This is a faint impressed line, parallel to the border of the valve and a little way within it, representing the line of attachment of the muscles that are placed near the edge of the mantle. If it is continuously convex outwardly, the shell is said to be integripalliate. If it is more or less indented by a pallial sinus, the shell is sinupalliate.

Pallial Sinus.-An infold of the posterior portion of the pallial line, sometimes a mere shallow curve, sometimes deep and extending back even beyond the centre of the valve. This occurs only in forms which can extend and retract their siphons.

The structure of the shell-substance itself exhibits two layers, the whole being covered in life by a skin, or "periostracum." The outer layer, sometimes thick, sometimes thin, consists of calcareous prisms in contact along their walls. Here and there a polygonal interspace occurs. The fibrous structure seen on cross-fracture of Inoceramus is due to well-developed prisms of this nature.

The inner layer is formed of delicate, compact, and pearly lamellæ, sometimes accumulated to a great thickness. These layers occasionally leave irregular interspaces or chambers of flattened and curving form, as in the thickened region near the umbos of some oysters.

The shell-substance is sometimes calcite, but most commonly aragonite. Or, when both minerals are present, the outer layer consists of calcite, the inner of aragonite. The mineral constitution of the shells of many genera yet awaits investigation, and the usual alteration of aragonite in old forms into granular calcite precludes certainty of determination in some extinct examples.

The lamellibranchiata are mostly marine; the fresh-water types referred to will be specially indicated. Some genera are attached to the sea-floor by the shell itself; others by fibrous outgrowths, the byssus, issuing near the umbos; others are free and locomotive. A few lamellibranchs, of different families, bore into mud, wood (as Teredo, the ship-worm), or into other shells, corals, or even the hardest stone (as Pholas and Lithodomus); the cavity thus made is called a crypt, and is increased until the animal and shell attain their full development. Thus the animal cannot leave the cavity, communication being kept up with the exterior through the narrow opening which repre

sents the first stage of the boring. The siphons are turned upwards, the anterior end of the animal being downwards, and a calcareous siphonal tube is sometimes developed, the small shellvalves becoming dwarfed by comparison and incorporated with the tube, and the whole shell thus appearing cylindrical. Aspergillum is one of the most remarkable examples.

Boring shells are often represented merely by casts of their crypts, which are often club-shaped, the short handle of the club being the result of the infilling of the narrow entry to the cavity.

Fossil siphonate shells are occasionally found-and should be looked for in the position in which they lived in the soft mud which ultimately entombed them; their umbos are thus directed downwards, and their siphonal ends upwards, in the stratum.

A. HOMOMYARIAN SIPHONATE FORMS WITH PALLIAL SINUS (SINUPALLIATE).

The adductor muscular impressions are two in each valve, one posterior, one anterior, and fairly equally developed. The animal possessed long retractile siphons. In certain exceptional families these siphons are encased in a calcareous tube projecting far beyond the limits of the valves.

Cytherea (fig. 69).-Shell thick, approximating to circular, umbo well forward, with lunule. Generally concentrically marked. Three diverging well developed cardinal teeth in each valve. An anterior lateral tooth in left valve. Pallial sinus acuteangled, moderately developed. Inner margin of shell smooth. Cretaceous to Recent.

Venus.-Like Cytherea, but without lateral tooth, and commonly with delicately grooved inner border.

Jurassic to Recent.

Tellina. Slightly inequivalve. Shell thin, elongated oval, rounded anteriorly, more acute behind. Umbos almost in centre. Concentrically marked. Hinge narrow; in each valve two cardinal teeth, and commonly an anterior and posterior lateral tooth. Sinus very broad and deep (fig. 70).

Cretaceous to Recent; this genus is particularly rich in living species.

Panopæa (Glycimeris).-Shell thick, often large, and approaching an elongated rectangle. Gaping at both ends. Umbos rounded and placed well forward. Concentrically marked. One cardinal tooth in each valve.