138 MAZATLAN BIVALVES 



Genus BYSSOAECA, Swains. 



Aj'ca, sp. typ. Linn., auct. — Cibota, Brmcn. — Daplme, Poli. — 

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

 Swain.'i. 1835.* 



189. Btssoabca Pacifica, Sow. 



Proc. Zool. Soe. 1833, p. 17. 



Area Pacifica, Hve. Conch. Ic. pi. 11. f. 75.— Hani. Pec. SMls, 



pi. 18. f. 59.-5. M. Cat. D'Orb. Moll. p. 8 2, no. 735. 



Tills maguificent 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 imiform outline and small pedal opening. The bj^ssal 

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

 jects 'beyond the sheU. Teeth extremely numerous, sharp, 

 generally narrow, sometimes rather broad. Ligament very 

 thin, disti'ibuted in a lozenge over the whole area except the 

 margin, with coarser lines radiating fi'om the imibos, and 

 sometimes others irregularly crossing these. The smallest 

 of the specimens measures long. '77, /«/. 1'4, alt. '8. 



A narrow sp. „ 2'2, „ 3'fi, ,, 2"8. 



The largest sp. „ 3', ,, 5'5, „ 4". 



This sp. measures inside from middle of hinge to ventral 

 margiu 1'3 ; from posterior end do. 2"4; projection of imibos 

 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, 



D'Orbignij. — Bijooga Is. Capt. Beaufort, B. M. — Mazatlan ; 



rare, but very fine ; L'pool Col. 



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

 to this species. 



Tablet 614 contains 2 sp. young. — 645, 2 sp. adolescent ; 

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

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

 mally broad. — 616, sp. not produced, incrusted with coral. — 

 647, the largest sp. — 618, stone incrusted with Brj'ozoa, with 

 byssal foot attached. 

 • Mr. A. Adams names chese shells .4reo, and the previous species Scapharea, Gray. 



