BRACHIOPODA. 356 
The valyes of the Brachiopoda are respectively dorsal and 
ventral; the yentral 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 
Terebratulidea. 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 
receiyed by sockets in the other; this hinge is so complete that 
the valves cannot be separated without injury.* A few genera 
haye no hinge; in Crania and Discina the lower valve is flat, 
the upper like a limpet; the valves of Lingula are nearly 
equal, and haye been compared to a duck’s bill. (Petiver.) 
Ventral valve. 
Dorsal valve. 
Fig. 145. Muscular system of Zerebratula.+ 
a a, adductor-muscles; 7, cardinal-muscles; x, accessory cardinals; p, ventral 
pedicle-muscles; p', dorsal pedicle-muscles; z, capsular-muscles; 0, mouth; v, vent: 
7, loop; t, dental socket. 
This and several other points of difference seem to show the 
propriety of adopting the proposal made by Deshayes in 1836 
of dividing the Brachiopods into two great groups, the one 
haying articulated, the other non-articulated valves. In the 
first, moreoyer, 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 the perivisceral cayity. 
This difference 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 + of an inch, except 
by applying force. 
+ Waldheimia Australis, Quoy. 4+ From a drawing by Albany Hancock, Esq. 
