326 INTRODUCTION TO CONCHOLOGY. 



scattered here and there upon the warm uniform ground 

 which covers the surface, characterizes the P.paUmm, or 

 Mantle Pectunculus, from Zanzibar. 



The genus Nuciila includes two separate and distinct 

 divisions of species. In one the shell is thin, and covered 

 with a green horny epidermis ; in the other it is thick and 

 solid, with a dark brown epidermis : the interior of the 

 latter differs also from that of the former^ in being pearly. 

 The umboes are contiguous, there being no intermediate 

 area, as in the rest of the Arcacea, in consequence of the 

 change in the position of the ligament. The hinge is 

 linear, with an internal obliquely-produced pit in the centre, 

 for the reception of the ligament ; the teeth are numerous, 

 sharp, rather recurved, set in a straight series on either 

 side the ligamentary cavity. The muscular impression of 

 the mantle is entire. 



While recurring to the species above mentioned, we 

 cannot avoid remarking the variety of structure and con- 

 figuration, conspicuous in each. No two shells are alike 

 in all their parts and tintings, and, doubtless, individual 

 members of the family would exhibit an equal difference 

 with respect to their forms and instincts, were it possible to 

 become perfectly acquainted with them. Nay^ more, we 



