BRACHIOPODA. 



653 



ventral valve is a subtriangular plate, which conceals the minute tubular 

 foramen. In the forms which are included in the sub-genus Orbiculoidea, 

 the general structure is the same as in Discina proper, but the valves are. 

 much elevated, and the minute foramen is situated at the posterior ex- 

 tremity of a groove, which is prolonged externally only, but has no inter- 

 nal continuation. The forms comprised in the sub-genus Discinisca, 

 again, possess a longitudinal foramen situated between the beak of the 



m 



Fig. 498. — Discina. {Or- 

 biculoidea) nitida, from 

 the Carboniferous rocks 

 of North America. d, 

 Dorsal valve ; v, Ventral 

 valve. (After Zittel.) 



Fig. 499. — The animal of 

 Discina, as seen on the re- 

 moval of the ventral valve and 

 part of the ventral mantle- 

 lobe, p, Expanded surface of 

 the peduncle. (After Wood- 

 ward.) 



Fig. 500. — Ventral 

 valve of Discina Circe, 

 from the Trenton Lime- 

 stone (Ordovician) of 

 Canada, showing the 

 pedicle - groove. (After 

 Billings.) 



ventral valve and its hinder margin, in the centre of a depressed oval 

 disc, the valve itself being flat or concave. The species of Discina, in 

 the general sense, range from the Ordovician to the present day. In 

 the genus Trematis (fig. 495, a) both the valves are more or less convex, 

 and the general shape of the shell is sub-orbicular or transversely oval. 

 The dorsal valve is the most convex, its beak being marginal. 



false 



ith a 

 area " below it. The ventral valve is flatter, and has a wide mar- 



3y 



Fig. 501.— Crania Ignabergensis. Cretaceous. 



ginal slit for the passage of the peduncle. The surface is radiately 

 striated, and is ornamented with small superficial pits. The species 

 of this genus are Ordovician and Silurian. 



Family 4. Craxiad.e. — In this family the shell is calcareous, 

 more or less circular in shape (fig. 501), the ventral valve being 

 the shallowest, and being usually fixed by the substance of the test 



