152 PERMIAN FOSSILS. 



Class Lamellibranchiata, Blainville. 



DiTHYEA, Aristotle. 

 AcEPHALES Testac£s, Cuvier. 

 CoNCHTFERA, Lamarck. 

 Bivalvia, Fleming. 



Diagnosis. — '^The Lamellibranchiata are bivalve conchiferous Mollusca, which 

 respire by gills in the form of muscular plates of membrane attached to the mantle." 

 (Owen.)^ 



This extensive group of what may be termed ordinary bivalve shells, is divisible 

 into two sections, depending on their valves being attached to each other by one or 

 two adductor muscles. 



Order Monomyaria, Lamarck. 



M^SOMYONES, Latreille, 1825, 



Diagnosis. — The valves attached to each other by a sub-centrally situated muscle. 

 The present section, of which the common oyster is a familiar example, contains the 

 following as its Permian representatives. 



Family Pectenid^ {Les Pectinides, partini), Lamarck. 



With the exception of Anomida and Ostreidce^ this family is the lowest Hn classifi- 

 cation of the present class, and widely distinguished from those named in several 

 important particulars ; even obviously from the last one, in which it is often placed ; 

 as its superior valve (the notched one in Pecten, the attached one in Spondylus) is inferior 

 in OstreidfB : it does not appear to be sufficiently known that, in this respect, both 

 Anomidce and Ostreidce differ from all other Monomyarians. 



Of the genera included in this family, the two following are all that are known to 

 occur in the Permian rocks of England. 



Genus Pecten, Miiller. 



Diagnosis. — " Shell sub-orbicular ; beaks approximate ; ligament internal, seated 

 in a triangular cavity, a byssus issuing under the ear of the right valve ; foot small, 

 pedunculated ; mouth with branched tentacula," (Fleming.)^ 



The present genus has existed from the Devonian period to the present time. 



^ Lectures on Comparative Anatomy, vol. i, p. 269. 

 2 British Animals, p. 383. 



