BRACHIOPODA ; TEREBRATULA. 



421 



Lingula, this attachment is effected by means of a fleshy tubular 

 footstalk ; and this footstalk or peduncle passes out, in the 

 Terebratula and its allies, through an aperture or notch in the 

 beak of the shell (Fig. 600). In Orbicula, on the contrary, the 



FIG. 599. TEREBRATULA 

 PSITTACEA. 



FIG. 600. CONVEX VALVE OF 

 TEREBRATULA. 



peduncle is wanting : and the lower valve of the shell itself becomes 

 the medium by which the attachment of the animal to the rock 

 is accomplished. There is thus the same kind of difference 

 between these genera, as there is between the pedunculated and 

 sessile Cirrhopods, a Class, of which we are also strongly 

 reminded by the structure of the arms of the Brachiopoda. 



966. The greater part of the existing Mollusks included in 

 this Class, belong to the genus Terebratula; of which about forty 

 species are at present known to exist, but of which several 

 hundred fossil species have been enumerated. These present 

 themselves in the very oldest rocks; and may be found, in 

 greater or less abundance, in almost all marine deposits, down to 

 the present time. The two valves in Terebratula are unequal, 

 one being nearly flat and the other convex ; it is in the latter, 

 that the opening is found for the transmission of the pedicle ; 

 but the former is still more remarkable for the curious internal 

 apparatus which is connected with it. This consists of a sort of 

 slender framework of shelly substance ; which projects consider- 

 ably into the cavity of the shell, and to which the arms just now 

 mentioned are attached. This framework is often found in 

 fossil shells, in a beautiful state of preservation. The arms of 

 the ordinary Terebratulae do not appear to be extensible beyond 

 the shell; but in the Terebratula psittacea (which is probably to 



