152 PERMIAN FOSSILS. 
Class LAMELLIBRANCHIATA) Blainville. 
Ditruyra, Aristotle. 
Ac&pHaes Trstacks, Cuvier. 
ConcuHtIFERA, Lamarck. 
Bivatvia, 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 MonoMyYArtA, Lamarck. 
Mtsomyongs, Latreéille, 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 PECTENIDE (Les Pectinides, partim), Lamarck. 
With the exception of Anomide and Ostreide, this family is the lowest in 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 Ostreide: it does not appear to be sufficiently known that, in this respect, both 
Anomide and Ostreide 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 Pectfen, 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. 
1 Lectures on Comparative Anatomy, vol. i, p. 269. 
2 British Animals, p. 383. 
