670 



M0LLUSC0IDEA. 



the ventral valve and a corresponding fold in the dorsal valve, and 

 having the beak of the ventral valve acute, incurved, and prominent. 

 The foramen is situated beneath the beak, open to view or con- 

 cealed, and entirely or partially completed by a deltidium. The 

 numerous species of Rhynchonella begin in the Ordovician rocks, 

 and the genus is well represented in the Silurian, Devonian, and 

 Carboniferous rocks. In the Secondary series the genus exhibits a 



Fig. 534. — Rhynchonella ca/>ax ; dorsal, profile, and ventral views. Ordovician. 



marked development in the Jurassic period ; but there are few Ter- 

 tiary species, and only six living forms are known. 



The Permian Brachiopods which have been included under the name 

 of Rhynchoporina resemble Rhynchonella in all essential respects, but 

 the shell is punctated. In the Silurian genus Eato7iia, hitherto only 

 found in North America, the teeth in the ventral valve are prolonged 

 into elevated ridges which enclose the muscular impressions, these latter 

 being divided by a median septum, a more developed septum being pre- 

 sent in the dorsal valve as well. In the Triassic genus Dimerella, 

 again, there is a large triangular foramen beneath the beak of the ventral 

 valve, and the dorsal valve has a prominent median septum which divides 

 the umbonal half of the cavity of the shell into two chambers. In Leio- 



Fig. 535- — Leiorhynchns Htironensis, viewed dorsally («), ventrally (i), and in profile (c). 

 Devonian. (Original.) 



rhynchus (fig. 535) are various Devonian Brachiopods, very closely allied 

 to Rhynchonella proper, but having the plications of the shell obsolete 

 on the lateral angles, while well marked on the mesial fold and sinus. 

 The beak of the ventral valve is pierced by a foramen, in at any rate the 

 early stages of growth, and there is a well-defined septum in the dorsal 

 valve. Triftlesia (Silurian) has a triangular pedicle-notch beneath the 

 beak of the ventral valve, the hinge-line being straight, with a well- 

 defined hinge-area ; while the cardinal process in the dorsal valve is pro- 

 minent and bifurcated, and the shell itself is trilobate. In the Ordovician 



