BEACHIOPODA. 



355 



Tiie yalves of tlie Brachiopoda are respectively dorsal and 

 ventral ; the ventral valve is usually largest, and has a pro- 

 minent beak, by which it is attached, or through ■which the 

 organ of adhesion passes. It is sometimes perforated, as in the 

 Terebratulidae. The dorsal, or smaller valve, is always free 

 and imperforate. The valves are articulated by two curved 

 teeth, developed from the margin of the ventral valve, and 

 received by sockets in the other ; this hinge is so complete that 

 the valves cannot be separated without injury.* A few genera 

 have no hinge ; in Crania and Discina the lower valve is flat, 

 the upper like a limpet ; the valves of Lingula are nearly 

 equal, and have been compared to a duck's bill. (Petiver.) 



Ventral valve. 



Dorsal valve. 

 Fig. 145. Muscular system of TerebratulaA 

 a a, adductor-muscles ; r, cardinal-muscles ; x, accessory cardinals ; p, ventral 

 pedicle-muscles ; jDf, dorsal pedicle-muscles; z, capsular-muscles ; q, rriputh; v, vent: 

 I, loop ; t, dental socket. 



This and several other points of dijEference seem to show the 



propriety of adopting the proposal made by Deshayes in 1836 



of dividing the Brachiopods into two great groups, the one 



having articulated, the other non-articulated valves. In the 



first, moreover, the valves are opened by muscles acting on the 



cardinal process of the dorsal valve, while in the latter the valves 



are opened by the pressure of the fluid in tlje perivisceral cavity. 



This difierence is accompanied by a striking variation in the 



the same with those in Mr. Davidson's Introduction, and in the British Museum 

 Catalogue. They are from original studies by the author, unless otherwise stated. 



* The largest recent Terebratula cannot be opened more than -1- of an inch, except 

 by applying force. 



t Waldheimia Australis, Quoy. -f-. From a drawing by Albany Hancock, Esq. 



