48 PALAEONTOLOGY OF THE UPPER MISSOURI. 



CARBONIFEROUS AGE. 



(PEBMIAN PEEIOB.) 



MOLLUSCA. 



CLASS LAMELLIBRANCHIATA. 



FAMILY PECTINIDJ3. 



Shell suborbicular, inequivalve, nearly or quite equilateral, very slightly 

 oblique, and more or less distinctly auriculate ; not nacreous, and without 

 a prismatic structure. Hinge line straight ; ligament marginal. Carti- 

 lage generally confined to an internal pit or groove under the beaks ; 

 sometimes occupying linear furrows in a cardinal area. Anterior margin 

 of one valve more or less deeply notched, or sinuous, for the passage of 

 the foot or byssus. Scar of the adductor muscle large, and subcentral ; 

 pallial line simple. 



Animal with mantle freely open and provided with double margins, the 

 inner one of which is fringed with pendent filaments, and the outer 

 bounded by a row of distinct ocular dots, or rudimentary eyes. Palpi 

 smooth externally, and pectinated on their inner sides ; mouth surrounded 

 by foliaceous leaflets. Gills equal, each pair partially folded upon itself. 

 Foot small, cylindrical, grooved, forming a byssus while the animal is 

 young. Sexes united. 



The Pectinidce are closely related to the Spondylidce, which form a natural 

 transition to the Ostreidce. They differ, however, in the more regular form of 

 the shell, and in never being attached by the substance of the valves. They also 

 generally want the strong interlocking teeth of the typical SpandylidcB, and the 

 animal differs in having a more developed foot and numerous ocular dots. 



In order to include in this family some extinct forms apparently belonging here, 

 it seems to be necessary to admit at least two distinct sub-families, distinguished as 

 below-: 



1. Pcctiuinae. 



Shell with anterior ear usually a little larger than the other ; cartilage occupying an internal pit or groove 



under the beaks. 

 Includes the genera Pecten, Vola, Camptonectes, Amussium, Syncycloncma, Hemipecten, and many undescribed 



living and extinct genera. 1 



1 We have been informed by Prof. Agassiz that he has recently studied this family with much 

 care, and that he finds it necessary to establish numerous new genera, which he has not yet published, 

 making, with those already established, more than fifty distinct genera, living and extinct. 



