72 BBACHIOPODS. 



moderate height, flat, and directed obliquely backward nearly 

 at right angles to that of the other valve ; beak very small, 

 scarcely distinct from the margin of the area, and minutely 

 perforated ; foramen broadly triangular, and arched over above 

 by the pseudo-deltidium, which is very deeply sinuous on its 

 inner edge, the sinus being nearly or quite closed by the dental 

 process and pseudo-deltidium of the other valve. 



Interior with cardinal margin somewhat carinate within ; 

 hinge teeth moderately prominent, remote and widely diver- 

 gent; dental ridges obscure and extending obliquely outward 

 and forward, but not produced or curving to surround a saucer- 

 shaped cavity for the muscular scars ; scars of adductor mus- 

 cles narrow, long and closely approximated or almost in con- 

 tact ; those of cardinal muscles on each side very large, fan- 

 shaped but shallow, separated sometimes by a small ridge in 

 advance of the adductor scars, and marked by radiating furrows 

 and ridges ; while the anterior and lateral regions are usually 

 marked by striae and scattering granules. Surface of both valves 

 ornamented by numerous radiating striae, that increase in num- 

 ber, on the ventral valve, mainly by intercalation, and are usu- 

 ally arranged with one to six or eight smaller and shorter ones 

 between each two larger and more prominent ones, the largest 

 one of which often occupies the mesial line ; while on the 

 dorsal valve they more frequently increase by division, and are 

 generally of more uniform small size. On well-preserved speci- 

 mens all the radiating lines are crossed by numerous very 

 minute, regular, closely arranged concentric striae, that are 

 invisible without the aid of a magnifier; a few moderately dis- 

 tinct, sub-imbricating marks are often seen near the free mar- 

 gins of adult shells. ( Meek.) 



Horizon and localities. Lower Silurian, Hudson shales: 

 Cape Girardeau. 



