MOLLUSCA. 



erebratula Digoua. Strophomeri.i Rugosa. 



Terebratula. 



a Valve with the spiral arms, b Valve with arins 



removed Terebratula digona. 



Producta Martini, 



Strjgocephalus Burtini. 



Terebratula Dorsata. 



Terebratula Alata. 



Terebratula Deformis 



Besides the orders now described as composing the class Conchifera, there is a very curious group 

 which should also be included in it, although established as a separate class by many naturalists. This 

 is the group of Brachiopoda, containing only three genera at present known, namely, Terebratula, Lin- 

 ffitla, and Orbicula; but formerly of much greater comparative importance. These animals have all 

 bivalve shells, differing in no essential particular from those of the Conchifera in general. The two 

 former genera are attached, however, by a footstalk proceeding from an opening in one* of the valves 

 near the hinge, to solid substances. The Orbicula is attached, like an oyster, by one of its valves. 

 But in the complexity of the muscular apparatus provided for giving motion to the valves, they much 

 surpass the highest of the other Conchifera. There are not only several muscles provided for the 

 closure of the shell, but another set to open it — an organization which no other bivalves possess. The 

 most peculiar part of their structure, and that from which they derive their name, consists in the 

 presence of two very long arms or tentacula, between the origin of which the mouth is situated. 

 These can be projected to a considerable distance from the shell, or drawn in and coiled up spirally 

 within it. They do not appear, however, to seize upon prey; but rather by means of the cdia with 

 which they are fringed, to i urrents which may bring food to the mouth. In the Terebratula, 



these arms are affixed at their irises to a very curious framework within the shell, the use of which is 

 uncertain; but it is believed to aid, by its elasticity, in separating the valves from each other. This 

 framework is most complex in the species in which the arms are shortest. The species of Brachio- 

 poda at present known, live at great depths in the ocean ; and many of their peculiarities seem to 

 have reference to that particular condition. They are distributed through all latitudes. 



We furnish above a few specimens of this group of shells. (378) 



