170 PALEOZOIC TIME. 



ticulates by Worms and Crustaceans. No evidence has been yet found 

 of the existence of Polyps (corals), among Radiates; or, in the earlier 

 epoch, of Lamellibranchs (ordinary bivalves), among Mollusks. 



e. The hinge-line may be straight, or not ; as long as the greatest breadth of the shell 

 (221, 229, 232), or shorter (227, 228). 



f. A cardinal area (hinge-area) may exist, or not; there is a large one in Fig. 221, and 

 none in Fig. 238. 



g. There may be a deltidium, — composed of one or two accessory pieces occupying 

 a triangular opening under the beaks, as seen in Fig. 224. Sometimes a similar open- 

 ing at the middle of the hinge is partly or entirely closed by the growth of the shell, 

 so as to leave a triangular prominence, called a pseudo-deltidium, as in Cyrtia, Strepto- 

 rhynchus, etc. 



h. The markings on the inner surfaces of the valves are of special importance, and 

 particularly the muscular impressions, usually situated near the median line, not far from 

 the hinge: on the dorsal (or smaller) valve there are, in the articulated genera, two pairs 

 (a and a' in Figs. 227, 230, 234, 236), sometimes coalescing so as to be one pair, for the 

 attachment of the adductor muscle (closing the shell): one is usually in advance of the 

 other, but in Figs. 230 and 233 they are side by side; on the ventral (or larger) valve, 

 there is a single impression on the median line between two others (Figs. 228,234); 

 the single impression is the insertion of the adductor muscle (a, Figs. 228, 231, 234, 

 237), and the pair are the insertions of the cardinal muscle; the latter muscle terminates 

 on the dorsal valve, usually in a small process. 



Families of Brachiopods. 



Terebratula Family (Figs. 150, 218-220). — Having arm-supports of the form of a 

 loop, attached to the smaller or dorsal valve, and a foramen at the apex of the beak. 

 Shell-structure punctate. 



Spirifer Family (Figs. 221-225). — Having spiral supports, shell usually with a 

 median fold; hinge-line commonly long and straight (sometimes short); beak large and 

 full. 



Rhynchonella Family (Figs. 226-228). — Having the arm-supports short curved pro- 

 cesses; beak usually full, but narrow, having a foramen; shell seldom wider than 

 high. 



Orthis Family (Figs. 229-237). — Arm-supports wanting; shell rarely with a median 

 fold; shell varying between orbicular and D-shape; beak usually very small, but some- 

 times produced. 



Productus Family (Figs. 238-240). — Arm-supports wanting; shell without a median 

 fold, or almost wholly so ; hinge-line straight, often as long as the breadth of the shell, 

 or nearly so, and without a cardinal area, or with only a narrow one (excepting Stro- 

 phalosia and Aulosteges) ; surface often tubular-spinous ; form usually D-shaped, with the 

 dorsal valve very concave; beak often very large and full. 



Discina Family (Figs. 243-245). —Thin and small disk-shaped shells; orbicular or 

 ovate; a slit or foramen through the ventral valve; no articulation between the valves. 



Lingula Family (Figs. 151 and 246). —Thin and small shells; orbicular or subovate; 

 no foramen; no articulation. 



Besides these, there are also the Crania and Tkecidium families. 



Genera of Brachiopods. — 1. Terebratula Family. — Genus Terebratula, like 

 Figs. 150 and 218; the loop small, as in Fig. .219. Genus Waldheimia, the same; the 

 bop large, Fig. 218. 



Besides these genera, Terebratulina has the side (or " crural") processes near the 

 base of the loop united (Fig. 220). Another genus, Terebratella, has the sides of the 

 loop united at middle by a cross-piece, and this piece soldered to the shell. Terebri- 

 rostra has the beak extravagantly prolonged, so as to be longer than the dorsal valve. 

 Rensselaeria has, instead of a loop, a peculiar hastate brachial support, projecting far 



