Note.-The limits of this genus are somewhat obscure.

Cainozoic. Many older forms referred to Panopæa are now placed with the Pholadomyidae (Gresslya, &c.)

Pholadomya.-Shell thin, elongated or obliquely oval, markedly convex; gaping behind and sometimes in front. Anterior border a little truncated. Umbos well forward. Escutcheon sometimes present. Marked with knotty radial ribs, particularly on the anterior surface; also with more delicate concentric lines. Practically toothless, one obscure process occurring in each valve. Sinus broad and fairly deep. The thinness of the shell makes casts alone commonly met with.

Lias to Recent. Characteristically Jurassic.

Goniomya.-Like Pholadomya, but marked with rather delicate ribs, forming Vs, the angle of which is directed towards the middle of the ventral border.

Especially Jurassic.

Homomya.-Like Pholadomya, rather elongated, gaping at both ends, but with only concentric striations. Trias to Cretaceous.

Gresslya. Also one of the Pholadomyidæ. Elongated oval, much like the longer Pholadomyas, but right valve somewhat larger than left, the umbo rising higher. Umbos well forward; lunule present, no escutcheon. Concentrically marked. Νο teeth. Right valve with a ridge running along the hinge-line from the umbo posteriorly, which leaves a furrow in the casts that frequently occur. Compare Ceromya.

Trias to Jurassic.

Ceromya (Isocardia in part).-Inequivalve, sometimes the right, but more commonly the left valve being slightly the larger, the umbo rising higher, as in Gresslya, and the posterior border overlapping that of the other vaive. Approximating to circular, strongly convex, slightly gaping. Umbos large and well rounded; lunule feeble or absent. Concentrically marked. No teeth. Ridge in right valve, as in Gresslya. Commonly found as casts. Typically Middle and Upper Jurassic.

Mactra. Shell fairly thick, approximately triangular, rounded in front, more pointed behind; gaping slightly posteriorly. Concentrically marked. A cardinal tooth in each valve, bifurcating, and thus shaped like an inverted V; behind it, and still under the umbo, a triangular pit, which marks the position of the cartilage or internal ligament. A second cardinal tooth, of lamellar shape, is sometimes present. Anterior and posterior lamellar lateral teeth well marked, those of the right valve being

double-i.e., consisting of two parallel ridges running along the hinge-line. Sinus shallow.

Middle Jurassic to Recent. Especially Cainozoic.

Mya (fig. 71).-Inequivalve, left valve the smaller. Elongated, somewhat oblong; gaping markedly at both ends. Umbos

Concentrically marked. Left

approaching centre of margin. valve with a well developed spoon-like process under the umbo, for the attachment of the cartilage. Right valve with one small cardinal tooth. Sinus large and deep; pallial line often shows strong subsidiary impressions running upwards from it.

The Myas burrow into sandy mud, particularly near the shore. Miocene to Recent.

Corbula (fig. 72).-Ally of Mya. Inequivalve, left valve much the smaller. Shell small, oval, produced posteriorly, ending there with rather a straight border; not gaping. Concentrically marked. One cardinal tooth in right valve; left valve with a process much like that of Mya, which fits into a groove behind the tooth of the right valve. Sinus quite shallow. Marine or Estuarine.

Trias to Recent. Especially Cainozoic.

Leda. Shell small, elongated and narrowed posteriorly, with umbos directed backwards. Hinge-line bent, with numerous transverse teeth, as in Nucula, with which Leda should be compared (see fig. 80).

Gotlandian to Recent.

Teredo. One of the Pholadida; the so-called ship-worm. Shell small, each valve three-lobed, the central lobe the longest. Concentrically striated. No true hinge or ligament, the valves being quite subsidiary to the great siphonal tube, which extends far beyond them.

The borings of this mollusc are tubular like those of worms, but typically somewhat straighter; they are found commonly in fossil wood, as in the London Clay, either empty or infilled with mud or crystalline deposits.

Lias to Recent. Especially Cainozoic.

B. HOMOMYARIAN SIPHONATE FORMS WITHOUT PALLIAL
SINUS (INTEGRIPALLIATE).

The adductor muscular impressions are two in each valve, as in the preceding group, and the absence of the pallial sinus makes the interior of the valves resemble those of homomyarian asiphonate forms. The siphons of the animal were not retractile. Cardium. The common Cockle. Shell fairly thick, approximately circular, or elongated in a vertical direction; sometimes slightly gaping behind. Umbos rather large and rounded. Radially ribbed, the ribs commonly ornamented with protuberances. Two cardinal teeth and an anterior and posterior lateral in each valve (fig. 73). Inner border notched.

Forms with radial markings on the posterior part only, and concentric on the remainder, have been sometimes divided off under the name Protocardia.

Rhætic to Recent; especially Cainozoic.

Conocardium (fig. 74).—Shell heart-shaped when viewed from

Fig. 73.- Hinge of Cardium edule (Recent). Left valve. c, The two cardinal teeth. 7, The ridge-like lateral teeth.

Fig. 74.-Conocardium aliforme
(Carboniferous).

the front, but somewhat elongated behind; strongly convex. The anterior end is truncated, the umbo being close to it; just below the umbo a tube-like prolongation of the shell runs out in continuation of the hinge-line. Gaping at the posterior end.

Radially marked; margins sharply folded by the ridges and furrows. Almost toothless.

Gotlandian to Carboniferous.

Cyrena (fig. 75).-Shell thick, oval, sometimes rather acute posteriorly. Concentrically marked. Three cardinal teeth and an anterior and posterior lateral in each valve.

Sometimes a

slight pallial sinus. In the sub-genus Corbicula the lateral teeth are elongated and transversely striated.

Brackish or fresh-water at present day, but often associated at river-mouths with typically marine shells, and hence of little value as a guide to conditions of deposition.

Lias to Recent.

Cyclas (Sphærium).-Ally of Cyrena, but shell thin and nearly equilateral. One feeble cardinal tooth in right valve and two in left. Lamellar anterior and posterior lateral teeth. Fresh-water. The reference of Wealden species to this genus has been criticised, though very generally made. Sandberger quotes the earliest Cyclas as Eocene, referring older forms to Cyrena, &c. Cyprina.-Shell thick, oval, resembling Cytherea; markedly convex. Concentrically striated. Two cardinal teeth and one posterior lateral tooth in each valve. There are also two feeble anterior lateral teeth in the right valve; and in the left valve one anterior lateral (Fischer).

Lias to Recent.

Astarte (fig. 76).-Shell thick, approaching circular at times, at others obliquely elongated. Umbos rather pointed; lunule generally present. Concentrically marked. Two cardinal teeth in each valve; no laterals. Lias to Recent. Especially Cainozoic.

Cardita Shell thick, somewhat like Cardium, but occasionally elongated. Umbos well forward. Radially ribbed. Inner border notched. Two elongated cardinal teeth, and a feeble anterior lateral, in right valve; two divergent cardinal teeth, and one feeble posterior lateral, in left valve. (As to difficulty in reading teeth in Cardita, see Fischer, Conchyliologie, p. 902.) Trias to Recent. Especially Cainozoic. Venericardia.-Much like Cardita. Two oblique cardinal teeth, and a feeble anterior lateral, in right valve; two cardinal teeth and one long lamellar posterior lateral in left valve. Interior of margin of shell distinctly notched.

Eocene to Recent.

Crassatella. Shell decidedly thick, oval, truncated slightly at posterior margin. Lunule present. Three cardinal teeth in right, two in left valve; an anterior and posterior lateral in both. Pit for cartilage immediately below the umbos, and behind the middle cardinal tooth of the right valve.

Cretaceous to Recent. Especially U. Cretaceous and Eocene.

Cardinia.-Placed by S. P. Woodward near Cardita, by other authors as an asiphonate form near Unio. Shell elongated, oval, flattened at the sides. Umbo well forward. Concentrically marked. Two cardinal teeth in left valve, one in right, their feeble development being a characteristic feature; anterior lateral tooth in right valve, elongated posterior lateral in left, well developed.

Jurassic. Especially Lias.

Chama.-Inequivalve; commonly attached by the umbo of the left valve, the upper valve, which is therefore generally the right, being the smaller. Shell thick, almost circular. Umbos of both valves bent forward and curved over, as if about to coil spirally. Concentrically marked, the successive shell-layers protruding beneath one another with irregularly serrated edges, giving the surface a foliaceous appearance. One stout cardinal tooth in upper valve, two in lower, between which the first-named fits. Lower Cretaceous to Recent. Especially Cainozoic.

Diceras. Slightly inequivalve; attached by umbo of one or other valve. Shell thick, approximately circular. Umbos very prominent, each being spirally curved and recumbent, as it were, against the surface of the shell. Concentrically marked. Right valve with one cardinal tooth, somewhat flattened and folded;