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48 MAZATLAN BIVALVES 
is very gibbous: sometimes subtriangular, and even taking the 
form of D. punctatostriatus. The lines of growth shew that the 
adult by no means thinks it necessary to preserve the form of 
early life: it being very common to observe the posterior part 
much developed in the young, while it is narrowed off in matu- 
rity ; or the shell is sometimes sub-oval when young, and very 
transverse afterwards. In colour there are similiar variations ; 
the young shell abounding more in purple, while the adults 
reserve that colour for the inside, and content themselves with 
a greenish gray. Orashell of an ochre yellow when young 
will change afterwards to a purplish tinge. The prevailing 
colour is a greenish gray, tinged with purple. This is very 
bright in the young, very dull in the adult. The purple is 
often absent in the adult, sometimes in the young; rarely 
predominant. Ochre yellow sometimes prevails, in the young 
beautifully shaded into purple or white. The white variety is 
tolerably common, with gradations into the other colours ; and 
often with irregular rays from the umbo to the anterior margin, 
which is but rarely seen in D. punctatostriatus. The same colours 
intensified are seen in the young shells, and noted by Hanl. in 
his description of D. culter. In these the changes in form are 
extraordinary ; some being nearly triangular, while others take 
an extremely elongated form, with a posterior ventral sinua- 
tion (D. culter, var. a, Hanl.) ; but the intermediate forms are 
so various that I have not been able to separate them. The 
middle stage of growth of the gibbous form is D. contusus, Rve., 
aspecies described from a single specimen in the Cumingian 
Collection. The surface of the shell is punctatostriate ; with 
the dots finer and the rows further apart than in the last species. 
Towards the ventral part they generally subside into plain 
strie. On the anterior part, they are often crowded and 
irregular, having jagged edges or running into little lines which 
are sometimes confluent. A large lunular portion is smooth. 
The general aspect of the shell is rather glossy, especially in 
the young shells. The form contusus often retains the margin 
marks one after another, giving the shell a tiled appearance. 
Having examined every specimen under the glass, 1am unable 
to note any constant characters in the markings co-ordinate 
with the changes of form and colour. The crenations are gene- 
rally simple, without the broad marginal band conspicuous in 
D. punctatostriatus, and not shewing the intricate markings at 
the anterior end. Ihave observed no specific differences in 
the interior: the teeth and impressions are as in D. punctato- 
striatus, making allowance for the greater or less elongation of 
