132 MAZATLAN BIVALVES 



Sab. Mazatlan ; 1 sp. in Spondylus ; L'pool Col. 

 Tablet 607 contains a sketch. 



FAMILY ARCADE. 



GENUS AECA, Linn. 



Area, pars, Linn. Lam. Area, Swains. 184). Senilia, Gray, 



1848. 



The Arose appear to divide themselves into two natural 

 groups, those which live freely in sand or mud, like cockles, 

 whose name they commonly bear ; and those which live nest- 

 ling in crevices or affixed to rocks by a horny byssoid pedal 

 appendage. The former have stout, regular, strongly ribbed 

 shells, and are considered by Swainson the typical species ; 

 the latter have irregular shells, generally with a thin or shaggy 

 epidermis, and a more or less developed ventral gape. These, 

 which are the typical species of most authors, form Swainson's 

 genus Byssoarca. 



- 180. AECA GEANDIS, Brod. fy Sow. 



Zool. Journ. vol. iv. p. 365. Rve. Conch. Ic. pi. 1, f. 4. Hani. 



Descr.. Cat. p. 160. B. M. Cat. D'Orl. Moll. p. 82, no. 729. 



C. B. Ad. Pan. Shells, p. 259, no. 417. 



This species, the W. Pacific analogue of the East Indian 

 A. senilis, is known from it by the greater number of ribs, 

 (25-30) and by the teeth which are comparatively narrow. 

 They are however wider than in A. tuberculosa. The shell 

 in its young state is generally subsequilateral, subquadrate, 

 often slightly insequivalve, with the epidermis for the most part 

 smooth and persistent, displaying here and there hairs, and 

 cancellating scales across the grooves. The ribs-are then often 

 granulose. As it advances to maturity, the posterior part 

 generally becomes much produced, till the aspect of the shell 

 becomes very insequilateral, and not unlike that of A. tuber- 

 culosa r . It may however always be distinguished from it by its 

 greater solidity, the smaller number of ribs, and by the grooves 

 which nearly equal the ribs in breadth. The epidermis grad- 

 ually becomes thick and rough, forming in concentric layers, 

 which are very conspicuous across the grooves. The umbonal 

 portion generally becomes decorticated. The ligament is 

 coarse and solid, filling up the whole of the hinge area except a 



