150 MAZATLAN BIVALVES 
Some confusion has arisen in this genus as in Pinna from a 
large number of the Malleus-shaped species from New Guinea 
having been brought at the same time with the Mazatlan 
collection, and having found their way into museums as though 
from this locality. he variations of form in these were most 
extraordinary. 
205. Isognomon CHEMNITzIANUM, D’Ord. © 
Perna Chemnitziana, D’ Ord. Sagra, Cuba, vol. ii. p. 346.— 
B. M. Cat p. 46, no. 547. 
Perna flexuosa, Sow. ms. in Coll. Kellett, Mus. Pract. Geol :— 
(do. Panama, Lieut. Wood, Bristol Mus.) 
= Perna, sp. ind. (a), C. B. Ad. Pan. Shells, p. 250, no. 393. 
Comp. Isognomon Perna, Linn. in Dkr. Moll. Guin. no. 116, 
pl. 8, f. 7-10:=(teste Dkr.) Ostrea perna, List. Conch. Tab. 
199, f. 33.—Chemn. Conch. Cab. vol. vii. p. 252, pl. 59, f£. 580.— 
Schroet. Einl. vol. ui. pl. 9, f. 5.—Lam. vii. p. 78, Perna 
vulsella, excl. var. 6. [Common at St. Vincent and Loander, 
Tams. “‘Omnia plane respondent ls, que ex Antillis insulis 
et ex America centrali originem ducunt.=sine dubio, Concha 
semiaurita, var. Chemmn. vol. vii. f. 580. (? f. 579,=P. radiata, 
Anton Verz. p.17).| Non O. semiaurita Schroet. loc. cit. f..6.” 
The Mazatlan shells are certainly the P. flexuosa of Sow. I 
cannot detect the slightest specific difference between these, 
and specimens collected at St. Vincent’s (W. I.) by Dr. W. B. 
Carpenter. These are the P. Chemnitziana of D’ Orb., which 
name I have therefore adopted till the date of Sow.’s is known. 
Whether it is the shell quoted by Dkr. from Central America 
and W.I. as well as from Guinea, cannot in so variable a 
genus be decided only from descriptions and figures. The 
Mazatlan shelis are small, very variable in shape, normally 
suborbicular, but generally produced. Shell with concentric 
irregular ridges of growth, but without sculpture on either 
valve. Beaks much produced, terminal; byssal portion incur- 
ved ; ventral part flattened. Muscular impression extremely 
large: ligamental pits deep, numerous and regular. Colour 
ash, more or less stained with purple, especially at the ventral 
part, often very rich. ‘The shell in its first appearance seems 
to be Cyrena-shaped, then like an inflated Anodon, then like 
TInoceramus, then for some time like a broad Pinna; then it 
gradually developes its true form, as far as 1s consistent with 
the crevice in which it has taken up its abode. One pit is de- 
