138 MAZATLAN BIVALVES 



GENUS BYSSOAECA, Swains. 



Area, sp. typ. Linn., auct. Cibota, Brown. Daplme, Poli. 

 Navicula, Blainv. 1818 (teste Gray.) non Spix Byssoarca. 

 Swains. 1835.* 



189. BYSSOAECA PACIFICA,. Sow. 

 Proc. Zool. Soc. 1833, p. 17. 

 Area Pacifica, Eve. Conch. Ic. pi. 11, f. 75. Hani Rec. Shells, 



pi. 18. f. 595. M. Cat. D'Orb. Moll. p. 8 2, no. 735. 



This magnificent species is known from B. Nose and its 

 congeners by its very large size, extremely swollen develop- 

 ment of the posterior portion, coarse ribs, large indentation 

 near the posterior hinge-angle, and the strong laminated gill- 

 like epidermis, which lies in very conspicuous striated folds 

 round the large byssal opening. It varies greatly in shape, 

 sometimes not displaying the above characters, but presenting a 

 tolerably uniform outline and small pedal opening. The byssal 

 foot has a hard horny case, grooved inside. It scarcely pro- 

 jects beyond the shell. Teeth extremely numerous, sharp, 

 generally narrow, sometimes rather broad. Ligament very 

 thin, distributed in a lozenge over the whole area except the 

 margin, with coarser lines radiating from the umbos, and 

 sometimes others irregularly crossing these. The smallest 

 of the specimens measures long. '77, lat.l'A, alt. '8. 



A narrow sp. 2'2, 3'6, 2'8. 



The largest sp. 3', 5'5, 4*. 



This sp. measures inside from middle of hinge to ventral 

 margin 1*3 ; from posterior end do. 2*4 ; projection of umbos 

 1*2 ; distance between umbos 1'8. 

 Hab.St. Elena ; on rocky ground, 6-18 fm., adhering to 



each other in large bunches ; Cuming. Ecuador, St. Elena, 



jyOMgny. Bijooga Is. Capt. Beaufort, B. M. Mazatlan ; 



rare, but very fine ; IS pool Col. 



Tablet 643 contains 1 valve, '03 across, probably belonging 

 to this species. 



Tablet 644 contains 2 sp. young. 645, 2 sp. adolescent ; 

 'Macrodon' teeth, branching from the centre, very conspicu- 

 ous on one side ; in the larger sp. the hinge teeth are abnor- 

 mally broad. 646, sp. not produced, incrusted with coral. 

 647, the largest sp. 648, stone incrusted with Bryozoa, with 

 byssal foot attached. 

 * Mr. A. Adams names these shells Area, and the previous species Scapharca, Gray. 



