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. Penta 



V 



nightii. The right-hand figur 

 tal plates of the shell. Upper 



l septa 



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- 

 lidce viz., Camarophoria and Porambonites, 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. STROPHOMENIDJE. 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 Strophomenida are exclusively Palaeozoic, and include the 



Fig. iS3-OrtAis Davitt- 

 toni; dorsal and side view. 

 Silurian. 



ponata; dorsal and 

 Silurian. 



genera Ort/u's, Orthisina, Strophomena, Lcfttana, and David- 

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

 need notice here. 



