258 TESTACEOUS MOLLUSCA. 
P. Ostiaua. Lam. 2.—Ostrea Atata. Gmel 3399.—D. p. 
283.—Knorr. 6, t. 21. f. 1.—O. Ata-corvi. Ch. f. 581.—O. Prr- 
wa. W.t.11. f. 78.—P. Epuiprium. Sow. G.—Reeve. Sys. t. 
106. f.2. Compressed, subovate, obliquely produced anteriorly, 
very fragile, purplish brown, but pale towards the beaks: hinge 
margin very short terminated anteriorly by an obtuse angle from 
which the margin runs in a scarcely incurved oblique line to the 
obtuse ventral, posterior sinus short and not deep: surface not stri- 
ated by radiating lines: edges very acute. 23. Antigua. 
P. Isocnomon. Lam 3.—Ostrea O. Lin 1149.—D. p. 282. 
Elongated, rather solid, dark chocolate brown, compressed, hinge 
margin forming anteriorly a longer or shorter lobe-like wing 5 sur- 
face often obsoletely subradiated by costellz ; internal nacre bluish 
white. Var. A.(Ch. f. 584.—W. t. 11. f. 79.—Bl. t. 63. fi 1. 
—E. t. 176. f. 1.—Son. G.f. 1.— Reeve t. 106. f. 1.) The body 
and elongated wing, nearly straight and nearly at right angles dor- 
sally, so that the shell presents a rude resemblance to a carpenters 
square. Var B. The wing almost obsolete. (P. Frmoratis. 
Lam, 5.—Ch. f. 582, 3.—Knorr 4.t 10. f. 1, 2.—H#. t.175. f. 4, 
5.) Var. C. (Seba 3. t. 91. f. 7.) Wing moderate, body obli- 
quely curving anteriorly. 6.  /ndian Ocean. 
P. Avicutaris. Lam.4.—Del t. 14. f. 3. Compressed, whit- 
ish, terminating ventrally in a broad oblique lobe: dorsal edge 
forming a short lobe anteriorly ; beaks conical and somewhat pro- 
duced: sinus for the byssus profound. Allied to the last. 
P. Canina. Lam. 6.—Seba. 3. t. 91. f. 8.—Knorr. 6. t. 18. 
f. 1. Compressed, triangular, rather broad dorsally, somewhat 
lobed on one side, violet brown: shell attenuated and elongated 
towards the ventral edge. 4. Indian Ocean and Australia. M. 
Deshayes considers this to be the young of Isognomon. 
P. Marsurium. Lam. 7. Ch. f. 577. Compressed, brownish, 
rounded ovate; posterior side with a deep sinus at its junction 
with the hinge which has but from six to eight teeth: nacre, violet 
within. Var.—Elongated and squarish: silvery within. New 
Holland. 
P. Sutcata. Lam. 8.—Ostrea Perna. Lin. 1149. Obovate, pale, 
yellowish, or the colour of rotten wood, with crowded elevated 
narrow ribs, which are rendered subnodulous by the layers of 
growth, the interstices lineated with a darker colour, usually ferru- 
ginous or purplish brown : hinge-margin rather short, anteriorly sub- 
auriculated, the anterior edge slightly convex in the young, in the 
adult nearly straight ; sinus for the byssus in one valve only, deep, 
short: ventral edge arcuated, internal nacre silvery white, cardinal 
sulei few. 24..2. New Holland. This is not the shell which La- 
marck's synonyms refer to, but is described from an examination of 
