138 MAZATLAN BIVALVES 
Gents BYSSOARCA, Swains. 
Area, sp. typ. Linn., auct.—Cibota, Brown.—Daphne, Poli.— 
Navicula, Blainv. 1818 (teste Gray.) non Spix.—Byssoarca, 
Swains. 1835.* 
189. Byssoarca Pactrica, Sow. 
Proc. Zool. Soc. 1833, p. 17. 
Arca Pacifica, Rve. Conch. Ic. pl. 11, f. 75.—Hanl. Rec. Shells, 
pl. 18. f.59.—B. MW. Cat. D’ Orb. Moll. p. 8 2, no. 735. 
This magnificent species is known from B. Noe 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 oill- 
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.1°4, alt. °8. 
A narrow sp. beatae PA AO,', ash AO: 
The largest sp. ek wes 1D Dasa Ge 
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, 
D Orbigny.—Bijooga Is. Capt. Beaufort, B. M.—Mazatlan ; 
rare, but very fine; L’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 Arca, and the previous species Scapharca, Gray. 
