10 



[Assembly 



Genus Pal^arca;* 



Generic description. Shell equivalve, very inequilateral, ovate or 

 rhomboid-ovate, gibbous or ventricose ; umbones subanterior : ante- 

 rior extremity abruptly rounded ; posterior extremity broadly rounded, 

 or sometimes obliquely subtruncate : anterior muscular impression 

 very strongly marked ( usually a deep round pit ), the posterior impres- 

 sion obscurely defined ; anterior teeth 3, 4 or 5, oblique, placed 

 beneath or in advance of the umbones, with two or three remote 

 oblique posterior or lateral teeth ; palleal line simple ; ligament 

 external ; ligamental area often deep and wide, striated. 



The shells of this genus vary in the development of the ligamental area and 

 in the teeth of the hinge-line, the latter often becoming much thickened, and 

 some of them obsolete with age : the anterior muscular impression is very 

 deep and strong, while the posterior one is superficial, though the shell is 

 always thinner at that point, as frequently observed in worn specimens. 



Pal^arca ventricosa. 



Fig. 1. The interior of the right valve of this species, showing the anterior and 

 posterior teeth, the ligamental area and muscular impression. 



Fig. 2. The left valve of an older specimen, in which the anterior teeth appear to 

 have been partially obliterated by age. The ligamental area is pro- 

 portionally wider than in fig. 2. 



Fig. 3. A cardinal view of the exterior of the same species, showing the ligamen- 

 tal area. 



* This genus was proposed in 1847, and the description has been printed in the 

 Palaeontology of New -York, Vol. Ill, with the accompanying illustrations. 



