180 PALEOZOIC TIME — LOWER SILURIAN. 



pods, Gasteropods (p. 156), and Cephalopods (p. 155). Thus, all the 

 grand divisions of Mollusks were represented. This cannot be said 

 of any other of the four sub-kingdoms of animal life. Even at the 



small and little prominent (figs. 219, 220) ; may have an aperture or foramen 

 at apex (figs. 160, 213, 214), or not 



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

 the shell (211, 219, 222), or shorter (217, 218). 



/. The presence or not of a cardinal area (hinge-area) ; there is a large one 

 in fig. 211, and none in fig. 228. 



g. The presence or absence of a deltidiiim, — composed of one or two accessory 

 pieces occupying a triangular opening under the beaks, as seen in fig. 214. 

 Sometimes a similar opening 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, Strejytorhynchus, etc. 



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

 and particularly the muscular impressions usually situated near the medial line 

 not far from the hinge : on the dorsal (or smaller) valve there are in the arti- 

 culated genera two pairs (a and a' in figs. 217, 220, 224,226), 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. 220 and 223 they are side by 

 side; on the ventn&l (or larger) valve there is a single impression on the medial 

 line between two others (figs. 218, 224) ; the single impression is the insertion of 

 the adductor muscle (a, figs. 218, 221, 224, 227), 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 Brachioj)ods. 



Terebratula Family (figs. 160, 208-210). — 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. 211-215). — Having spiral supports ; shell usually with a 

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

 large and full. 



Rhyuchonella Family (figs. 216-218). — Having the arm-supports short curved 

 processes ; beak usually full, but narrow, and having a foramen ; shell seldom 

 wider than high. 



Orthis Family (figs. 219-227). — Arm-supports wanting; shell rarely with a 

 medial fold ; shell varying between orbicular and D-shape ; beak usually very 

 small, but sometimes produced. 



Productus Family (figs. 228-230). — Arm-supports wanting ; shell without a 

 medial 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 in Strophalosia and Aulostegcs) : surface often tubular- 

 spinous ; form usually D-shape, with the dorsal valve very concave ; beak often 

 very large and full. 



Discina Family (figs. 233-235). — Thin and small disk-shaped shells; orbicular 



