MAZATLAN BIVALVES 53 
lineis subobsoletis utrinque ad marginem dorsalem decurrentibus, 
aream rhomboideam ut in Arca describentibus ; anterius lineis 
lunulam magnam demonstrantibus : intus dentibus later- 
alibus subequalibus, tenuissime rugosis, non striatis : 
sinu pallit minimo. 
The shell differs from the typical species in its regular form, 
and 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 Rangia may be retained for this species. 
It has somewhat the external aspect of Mactra subtruncata, 
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 smus is extremely small. The lines of 
growth outside are often beaded, which is the more remarkable 
as the margin is sharp, not crenulated. It varies, like other 
bivalves, in being more or less swollen ventrally, 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 Veneride &c. which I have examined, 
instead of ranging under two heads, there is every conceivable 
gradation of form. Indeed one of our most accurate British 
malacologists denies the separation of sex in the Lamellibran- 
ehiata altogether: v. Clark, Moll. Test. Mar. Brit. p. 191. 
Dong. ‘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. 
Famity VENERID. 
The genera in this family are so slightly defined that shells 
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. Veneride, 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 the 
fry and the adult. Itis probable therefore that there are many 
errors in the young of the following species. 
