672 



MOLLUSCOIDEA. 



restricted to definite horizons, while some forms have a very wide 

 geographical range. Well-known Silurian species are P. Knightii, 

 P. oblongus (fig. 538), and P. undatus, while P. globus and P. acuto- 

 lobatus are Devonian, and the familiar P. galeatus is found in both 

 sets of deposits. 



In the typical forms of Pentamerus the shell is more or less globose, 

 the ventral valve is much the largest, and the median septum in the same 

 valve is very long. In certain Silurian species, however, which have 

 been placed in Pentamerus, and for which Mr Billings proposed the 

 name of Stricklci7idia ( = Stricklandinia\ the two valves (fig. 539) are 



Fig. 538. — Large specimen of Pentamerus oblongus. 

 Silurian. (Original.) 



Fig. 539. — Stricklandia. 

 Davidsoni, viewed sideways 

 and dorsallv. Silurian. 



(After Billings.) 



not very disproportionately unequal, the shell is often more or less de- 

 pressed, and in the ventral valve there is but a short mesial septum 

 which supports a V-shaped chamber beneath the beak ; whereas in the 

 dorsal valve there are only two short socket-plates. Pentamerella, Am- 

 phigenia, Gypidula, and Anastrophia are other generic or sub-generic 

 titles which have been proposed for forms more or less closely allied to 

 Pentamerus itself. The first three of these occur in the Devonian, but 

 the last is found in the Silurian deposits of North America. 



The genus Poi'ambonites occupies a doubtful position, being some- 

 times placed in the present family, and sometimes referred to the 

 Strophomenidce. In this genus the shell is globose and almost equivalve, 

 with a mesial fold and sinus in front ; and the beak of the ventral valve 

 has a small foramen. The hinge-teeth in the ventral valve are supported 

 by strong dental lamellse, which unite to form a small median septum. 



