208 



MOLLUSCA. 



lateral spaces. In the interior of the smaller or dorsal valve 

 are two longitudinal septa, which often form a chamber cor- 

 responding to and apposed to the median chamber in the ven- 



Fig. 152. Pentamerus Knightii. The right-hand figure shows the internal septa 

 and dental plates of the shell. Upper Silurian. 



tral valve. The Pentameri range from the Lower Silurian to 

 the Carboniferous inclusive ; but they are especially character- 

 istic of that portion of the Silurian series known as the Lland- 

 overy formation. They often occur in the greatest profusion, 

 and the species have in many cases an enormous geographical 

 range. 



The two other genera generally placed in the Rhynchonel- 

 HdcK viz., Camarophoria zndiPorambomtes, are of little impor- 

 tance. The former is confined to the Carboniferous and 

 Permian deposits, and the latter occurs exclusively in strata of 

 Lower Silurian age. 



FAM. VI. STROPHOMENID^E. Animal unknown ; some pro- 

 bably free others attached, during the whole or a portion of 

 their existence, by a muscular peduncle. No calcified supports 

 for the arms. Shell with a straight hinge-line and a low trian- 

 gular area in each valve. Shell-structure fibrous or punctated. 

 The Strophomenidce are exclusively Palaeozoic, and include the 



Fig. i^.-^Orthis David- 

 soni; dorsal and side view. 

 Silurian. 



Fig. i$$.Orthis porcata; dorsal and 

 side view. Silurian. 



genera Orthis, Orthisina, Strophomena, Leptana, and David- 

 sonia, of which the first four only are sufficiently abundant to 

 need notice here. 



