132 MAZATLAN BIVALVES 



Hah. — Mazatlan ; 1 sp. in Spondylus ; L'pool Col. 

 Tablet 607 contains a sketch. 



Family AECAD^. 



Genus AECA, Linn. 



Area, pars, Linn. Lam. Area, Swains. 1840.— Senilia, Gray, 

 1848. 



The Arcsc 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 iji 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 tj-pical species of most authors, form Swainson's 

 genus Byssoarca. 



180. Aeca geandis, Brod. Sf Soto. 



Zool. Journ. vol. iv. p. 365.— i?re. Condi. Ic. pi. 1, f. 4,.— Hani. 

 Descr. Cat. p. 160.-5. M. Cat. D'Orh. Moll. p. 82, no. 729.— 

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



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

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

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

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

 in its yoimg state is generally subaquilateral, subquadrate, 

 often slightly ina?quivalve, with the epidei-mis for the most part 

 smooth and persistent, displaymg here and there hau-s, and 

 cancellating scales across the grooves. The ribs are then often 

 granulose. As it advances to maturity, the posterior part 

 generally becomes much produced, tiU tlie aspect of the sheU 

 becomes very uisequilateral, and not imlike that of A. tuber- 

 culosa. 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 breadtli. The epidermis grad- 

 ually becomes thick and rougli, forming in concentric layers, 

 wliich 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 



