MAZATLAN BIVALVES 51 



white. The shape is very constant, only occasionally varying 

 in the shortening of the anterior portion. The posterior lateral 

 teeth are extremely small, the anterior of moderate size. Pal- 

 lial sinus rather small and narrow. I have seen this shell 

 labeled as from the W. Indies, but without authority. It has 

 probably been mistaken for Lutraria carinata, teste Gould, 

 which he gives as the Caribbsean analogue of Lutraria ventri- 

 cosa, ( = Mulinia ventricosa, C. B. Ad. Pan SJiells, p. 293,) 

 which is given from Panama and Mazatlan. ? Can this be the 

 M. exoleta of Gray. Y. Proc. Bost. Soc. N. H. iv. 89 ; a work 

 which, alas ! I have not been able to see. In the very young 

 shell, the anterior margin is but little produced, and the pos- 

 terior tooth is moderately large. 



Youngest specimen measures long. *16, lat. '21, alt. *1. 



The most ventricose 2'8, 3'73, 1'86. 



The largest 3'84, 5*33, 2*5. 



Hob. Cape Horn [?] and West Columbia, Reeve. Guayaquil, 

 Hinds. Mazatlan ; not uncommon, L'pool Col. 

 Tablet 211 contains 1 very young valve. 212, 3 specimens ; 

 one young, of exquisite beauty ; one with the epidermis re- 

 moved, shewing the interior (presented by J. Hibbert, Esq.) ; 

 the other very large. 



79. MACTEA (SPISULA) FBAGILIS, Ckemn. 



Conch Cab. vol. vi. p. 236, pi. 24, f. 235. .#. M. Cat. D'Orb. 



Moll. p. 56, no. 489. #. M. Cat. Sagra Moll. p. 35, no. 418. 

 = Mactra ovalina, Lam. An. s. Vert. ed. Desk. vi. 104, no. 21, 



(teste Gray.) 

 =Mactra Braziliana, Lam. An. s. Vert. ed. Desk. vi. 106, 



no. 27, (teste DesJi.) 

 =Mactra oblonga, Say, (teste live.) 



Mr. Cuming having compared this shell critically with both 

 larger and smaller specimens in his collection, writes that it 

 certainly belongs to M. fragilis, Ckemn. (according to Desk.) 

 The texture and epidermis are very different from M. velata, 

 Phil, of which, for geographical reasons, I had thought it 

 might be the young. It is remarkable for its double posterior 

 ridge, great flatness, and large gape on each side. Long. 1*36, 

 lat. 1-93, alt. '64. 



Hob. W. Indies, Honduras and St. John's, Mus. Cuming. 

 Cuba, Sagra. Rio Janeiro, Lalande, jun. (teste Lam.) 



