CUCULLiEA. 539 



external ; ligamental area narrow ; cartilage grooves 

 angular, concentric. Pallial impression entire. 



Trisidos, Bolt, Trisis, Oken u 



Ex. P. tortuosa, Linnaus, pi. 125, fig. 4, 4, a. 



There are but two species of Parallelepipedum as yet 

 described, from India and China. The shells are readily 

 known from those of the allied genera by the valves being 

 twisted. 



Species of Parallelepipedum. 

 semitortum, Lam. tortuosum, Linn 



Genus CUCULL^IA, Lamarck. 



Shell equivalve, inequilateral, trapeziform, ventricose, 

 closed in frout; surface of valves radiately striated, and 

 covered with a furfuraceous epidermis. Hinge with the 

 teeth dilated and bent down at the outer ends, divided into 

 a few large longitudinal plates; ligament external; liga- 

 mental area moderate, smooth. Pallial impression entire ; 

 posterior muscular impression on an elevated plate. 



Ex. C. concamerata, Martini, pi. 125, fig. 5, 5, a. 



In this genus the posterior muscular impression is 

 bounded by an elevated ridge. Two species only are 

 known, from China, Nicobar, and the Mauritius. Fossil 

 examples, forming the genus Macrodon of Lycett, are 

 found in the Secondary and Palaeozoic series. 



Species of Cucullcea. 

 concamerata, Mart. granulosa, Jonas. 



