154 CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 



consider, therefore, that such an opposite assemblage of characters is 

 united in one family, we almost fear to have adhered too strictly to the 

 classification of Lamarck in allowing it to remain. The method which 

 Gray has introduced of separating the Spondylidce , as having teeth, from 

 the Pectinida which are edentulate, might certainly be adopted with 

 advantage ; we still uphold the former, however, because these divisions 

 are so neatly linked the one to the other ; the transition from Pecten 

 to Spondylus through that species which has been distinguished by the 

 generic title of Hinnites, together with Plicatula, is as perfect as it well 

 can be ; and we moreover retain the family of the Pectinacea, on the 

 principle that no new data or phenomena, whether of form and variety in 

 the shell, or of habit or organization in the animal, have appeared since 

 it was established by Lamarck. 



The shell of the Pectinacea may be described as being either free, or 

 attached by one valve ; it is often inequivalve, somewhat irregular, and 

 generally auriculated at the base on each side. The ligament is either 

 internal or semi-internal, inserted in a central pit or groove, and the 

 animal is not always provided with a byssus. The Pectinacea are di- 

 vided into five genera, as follows : 



Pedum. Plicatula. 



Lima. Spondylus. 



Pecten. 



PEDUM, Bruguiere. 



Testa longitudinalis vel elongato-securiformis, compressa, insequivalvis, 

 subauriculata ; valvis in area trigona, setate crescente, ad basem 

 productis ; valva inferiori convexiuscula, lateribus refiexis, latere uno 

 prope ad cardinem, byssi causa profunde sinuato ; valva superiori 



