150 MAZATLAN BIVALVES 



Some confusion has arisen in this genus as in Pinna from a 

 large number of the Malleus-shaped species from JN"ew Guinea 

 having been brought at the same time with the Mazatlan 

 collection, and having found their way into museums as though 

 from this locality. The variations of form in these were most 

 extraordinary. 



205. ISOGNOMON CHEMNITZIANUM, D'Orb. 



Perna Chemnitziana, D'Orb. Saqra, Cuba, vol. ii. p. 346. 



B. M. Cat p. 46, no. 547. 

 Perna flexuosa, Sow. ms. in Coll. Kellett, Mus. Pract. G-eol : 



(do. Panama, Lieut. Wood, Bristol Mus.) 

 = Perna, sp. ind. (a), C. B. Ad. Pan. Shells, p. 250, no. 393. 

 Comp. Isognomon Perna, Linn, in DJcr. Moll. Guin. no. 116, 

 pi. 8, f. 7-10: = (teste DJcr.) Ostrea perna, List. Conch. Tab. 

 199, f. 33. Chemn. Conch. Cab. vol. vii. p. 252, pi. 59, f. 580. 

 Schroet. Einl. vol. iii. pi. 9, f. 5. Lam. vii. p. 78, Perna 

 vulsella, excl. var. b. [Common at St. Vincent and Loander, 

 Tarns. "Omnia plane respondent iis, quae ex Antillis insulis 

 et ex America centrali originem ducunt. = sine dubio, Concha 

 serniaurita, var. Chemn. vol. vii. f. 580. (?f. 579,= P. radiata, 

 Anton Verz. p. 17).] Non O. serniaurita Schroet. loc. cit. f. 6." 

 The Mazatlan shells arc 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 Z>' Orb., which 

 name I have therefore adopted till the date of Sow's is known. 

 Whether it is the shell quoted by DJcr. 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 shells are small, very variable in shape, normally 

 suborbicular, but generally produced. Shell with concentric 

 irregular ridges of growth, but without sculpture on either 

 valve. Beats 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 

 Inoceramus, then for some time like a broad Pinna ; then it 

 gradually developes its true form, as far as is consistent with 

 the crevice in which it has taken up its abode. One pit is de- 



