MAZATLA.N BIVALVES 53 



liTieis suhobsoletis utrinque ad marginem dorsalem decurrentibus, 

 aream rhomboideam ut in Area describentihiis ; anterius lineis 

 lunulam magnam demonstrantibus : hitus dentibus later- 

 (I I ibtis subcequalibus, tenuissime rugosis, non striatis ; 

 sinupallii minimo. 



The shell differs from the typical species in its regular form, 

 aud consequent size of the anterior tooth, which in G. cuneatus 

 is very short and bent up. If this be regarded as of generic 

 importance, the name Eaugia may be retained for this species. 

 It has somewhat the external aspect of Mactra siibtruncata, 

 but the rounded character of the hinge teeth shews its affinities 

 to be with Gnathodon. It has the aspect of a brackish water 

 shell, and the pallial sinus is extremely small. The lines of 

 growth outside are often beaded, which is the more rem^arkable 

 as the margin is sharp, not crenulated. It varies, like other 

 bivalves, in being more or less swollen ventraUy, and less or 

 more produced posteriorly. This constantly variable character 

 among bivalves can hardly be the result of sex, as in the 

 myriads of shells of Veneridse &c. which I have examined, 

 instead of ranging under two heads, there is every conceivable 

 gradation of form. Indeed one of our most accurate Britisli 

 malacologists denies the separation of sex in the Lamellibran- 

 chiata altogether: v. Clark, Moll. Test. Mar. Brit. p. 191. 

 Long. -82, lat. 1-08, alt. -55. 



Hab. — Mazatlan ; rare ; L'pool Col. 



Tablet 215 contains 4 specimens, the largest and the smallest, 

 and two intermediate ones shewing opposite forms of growth. 



Family VENEEIDiE. 



The genera in this family are so slightly defined that sheik 

 classed in different sub-families by one naturalist will belong 

 to the same genus of another. The most accurate divisions 

 are those proposed by Dr. Gray, which will be found in Desk . 

 B. M. Cat. Veneridcv, 1853. Many tiny shells were found 

 among the shell washings, which there has been great difficulty 

 in affiliating, from not having intermediate specimens, and 

 from the change of form and hinge characters between tlie 

 fry and the adult. It is probable therefore that there are many 

 errors in the young of the following species. 



