Devonian to Recent. Especially Mesozoic.

Terebratulina.-Like Terebratula in all essentials, but deltidium small and surface of shell delicately striated by grooves radiating from the apex.

Jurassic to Recent.

Magellania (Waldheimia). Not externally distinguishable from Terebratula, but brachial loop long, and a median septum in dorsal valve. Lias to Recent.

Kingena.-Allied to Terebratula; hinge-line straighter, and brachial loop united to a median septum.

Jurassic and Cretaceous.

Pygope.-A Terebratula in which, after a certain age, the lateral parts of the valves grow outwards and then reunite, leaving an aperture through the whole form; this comes finally to lie nearer to the beak than to the growing margin. In casts the vascular impressions are well seen.

Tithonian (U. Jurassic).

Stringocephalus (fig. 60).-Shell punctate, and resembling a wide Terebratula, but ventral valve with distinct area; deltidium and pseudodeltidium both present. Strongly developed median septum in ventral valve. Cardinal process long and curved, bifurcating at end to pass on each side of the septum in the

Fig. 60.-Stringocephalus Burtini (Devonian). Deltidium missing.

Fig. 61.-Rhynchonella. Viewed from below, showing the plicated junction of the closed valves.

opposing valve. Loop curving round parallel to and near the margin of the valve.

Devonian.

Known also in Gotlandian.

Rhynchonella (fig. 61).-Shell impunctate, rather triangular,

the margin on each side of the beak being straight and the outer margin curved. Ventral valve commonly infolded down the middle line, and dorsal valve bulged out to correspond; margins almost always bent into sharp folds, giving well marked radial ridges down the surface. Beak sharp and bent over downwards and even inwards; foramen below it, commonly surrounded by the deltidium (compare Terebratula). No loop, the crura alone being present.

Ordovician to Recent.

Pentamerus (fig. 62).-Allied to Rhynchonella. Shell impunetate, markedly inequívalve, and strongly convex; smooth or furrowed. Beak curved downwards; no deltidium. Median septum in ventral valve strongly developed, dividing on its free edge into two diverging septum-like dental plates, between which a little

chamber is thus formed, open at the end away from the beak. The dorsal valve has two septa, arising one on each side of the central line, which approach the dental plates. The remarkable size of these structures in proportion to the cavity of the shell causes it to break open easily along a surface formed by the ventral septum, one or other dental plate, and the corresponding dorsal septum. The septa can sometimes be traced as lines on the convex exterior of the shell (fig. 62). Casts show characteristic deep grooves in the place of these internal partitions.

Gotlandian and Devonian.

Camarophoria.-Like Rhynchonella, but with an internal structure resembling that of Pentamerus on a small scale; one septum in the dorsal valve, dividing on its edge.

Devonian to Permian; especially the latter.

Spirifer (fig. 63).-Shell impunctate, commonly with a median ventral furrow and dorsal ridge-like fold as in Rhynchonella; generally also marked with radial grooves. Hinge-line straight, often forming the longest dimension of the shell, and even causing ear-like expansions of the margin just below it. Ventral valve with prominent sharp beak, very commonly curved over; area triangular; foramen triangular, closed over in part by a pseudodeltidium. Dorsal valve with small narrow area; brachial spires present and highly developed, as may fairly often be seen on breaking open the shell (fig. 64). They occupy almost all the valve, their apices being directed outwards.

S. P. Woodward notes that silicified specimens occur in which

the spires may be freed by the use of acid from the matter that obscures them.

Gotlandian to Permian. Very abundant in species in the Devonian and Carboniferous.

Spiriferina (fig. 65).--Like Spirifer, but punctate, and with a median septum in the ventral valve. Typically smaller than Spirifer. Perforations can easily be seen with a lens, especially on slightly rubbed specimens.

Carboniferous to Lias; typically the latter.

Retzia.-Shell punctate; marked by strong radial ribs. Foramen, with deltidium under it, in ventral valve. in dorsal valve, much as in Spirifer.

Spires

The genus, in its usual wide sense, is Gotlandian to Trias. Meristella (formerly classed with Athyris).-One of the Spiriferida. Shell impunctate, smooth, and resembling in form a wide Terebratula, but without the foramen of that genus, Well marked median septum in dorsal valve; spires similar to Spirifer.

Gotlandian and Devonian.

Atrypa. Shell impunctate, and resembling Rhynchonella, but typically with a straighter hinge-line. Foramen in beak, which

is curved over; deltidium below; no area. Dorsal valve with large spires, their apices directed towards the central part of the inner surface of the valve, and thus nearly touching one another. Ordovician to Trias; especially Gotlandian and Devonian. Koninckina. Form somewhat like Productus; dorsal valve concave. Apices of spires directed outwards.

Trias of Alps.

Orthis. Shell punctate, commonly approaching a rectangular shape, the valves often almost equal, and both only slightly convex; marked with radial grooves in almost all cases. Hinge-line straight, but shorter than the greatest width of the valve; each valve with an area which is notched in the centre, the two triangular notches together forming the foramen. Strongly marked muscular and vascular impressions. Cardinal process not divided (in some allied genera it is furrowed); brachial crura present, but small, and neither loop nor spires.

L. Cambrian to Ca boniferous. An extremely abundant genus in the older Palæozoic.

Strophomena. The Strophomenidæ have received of late considerable revision, on account of variations in the internal characters of species previously grouped under the same genus. Strophomena itself now includes shells without crura (compare Orthis), and with ventral muscular area bounded by raised margin. Ventral valve concave, dorsal convex. (Example:Strophomena rugosa.)

Ordovician.

Leptæna.-Like Strophomena in general, but with dorsal valve concave, ventral convex, and broad shallow ventral muscular area. The edges of the valves are often bent sharply over in a dorsal direction. Flatter part distinctly wrinkled in concentric folds. (Example:-Leptana rhomboidalis.)

Ordovician to Carboniferous. Doubt hangs over all the species recorded from the Lias. Mun er-Chalmas finds that some possess brachial spires, and has referred them to a new genus, Koninckella (Bull. soc. géol. France, 3me. sér., t. viii., p. 279).

Rafinesquina.-Like Leptæna, but without the abrupt bend in the shell, and unwrinkled. (Example:-Rafinesquina (Strophomena) alternata.)

Ordovician.

Productus (fig. 66).-Shell punctate, the perforations being

* See Hall, Pal. of New York State, vol viii. (1892), &c.

produced in spines of various lengths. Not attached by a pedicle, as are the preceding genera, but free, or occasionally fixed by the spinose surface of the ventral valve.* Surface

Fig. 66.-Productus giganteus (Carboniferous).

sometimes smooth; more commonly ribbed, with hollow spines, set mostly in the neighbourhood of the hinge. Ventral valve strongly convex, with curving beak; dorsal valve concave. No foramen; no hinge-teeth. Hinge-line straight, sometimes forming the greatest width of the shell, with ear-like expansions;

sometimes shorter.

Devonian to Permian. Especially Carboniferous.

B. INARTICULATA.

Valves not connected by a hinge, being kept closed by the adductor muscles, and moved apart in a lateral direction by "protractor sliding muscles," so that the apices of the valves are made to diverge from one another sideways, instead of approaching one another on opening, as in the more common brachiopods, the Articulata.

Lingula.-Shell formed of alternating lamellæ of horny matter and phosphate of lime, the former predominating; flexible in modern examples; punctate. Almost equivalve, each valve shaped like a flat shovel, pointed at the beak, truncated on the opposite margin. Smooth, or marked by mere delicate concentric growth-lines. A pedicle emerged between the beaks of the valves.

Ordovician to Recent.

Lingulella.-Like Lingula, but with a vertical slit running from the beak of the ventral valve, probably to allow of the *See R. Etheridge, jun., Quart. Journ. Geol. Soc., 1876, p. 454, and 1878,

P. 498.