48 MAZATLAN BIVALVES 



is very gibbous : sometimes subtriaiigular, and even taking the 

 form of D. punctatostriatus. Tlie 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 

 nauch developed in the young, while it is narrowed off in matu- 

 rity ; or the sheU 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. Or a shell of an ochre yellow when young 

 will change afterwards to a purpUsh 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 yeUow 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 rai'ely seen in D. punctatostriatus. The same colours 

 intensified are seen in the young shells, and noted by Hani, in 

 his description of D. cidter. In these the changes in form are 

 extraordinary ; some being nearly triaugxdar, while others take 

 an extremely elongated form, with a posterior ventral sinua- 

 tion (D. culter, var. a, Hani.) ; 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. Hve., 

 a species described from a single specimen in the Cumingian 

 Collection. The surface of the shell is pimctatostriate ; with 

 the dots finer and the rows further apart than in the last sijccies. 

 Towards the ventral part they generally subside into plain 

 strise. On the anterior part, they are often crowded and 

 irregular, having jagged edges or running into little lines which 

 are sometimes confluent. A large hinular 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 luider the glass, I am 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. pimctatostriatus, and not shewing the intricate markings at 

 the anterior end. I have observed no specific difierences in 

 the interior : the teeth and impressions are as in D. punctato- 

 striatus, makmg allowance for the greater or less elongation of 



