MAZATLAN UNIVALVES 269 



My attempts to find specific differences between the Atlantic 

 and Pacific shells have entirelj- failed. The former are generally 

 of a more reddish, the latter of a browner cast ; but those 

 from Chili belong to the Atlantic type ; while those from 

 Honduras go through the same changes as the Mazatlan shells. 

 The Patagonian shells may belong to either type. The C. 

 echinus of Brod. represents the form in which all the ribs are 

 equal and very spiny ; the C. hystrix that in which a few are 

 developed, with large spines, at the expense of the rest. The 

 two forms run into one another, and into the common form 

 almost imperceptiblj'. In first describing them, JBrod. candidly 

 states that he would not be positive that they are not all varie- 

 ties of C. aculeata. There is a distinct variety which bears 

 the same relation to the t}*pical form that C. squama does to 

 C. nivea. It is flat, very regiilar, Tvithout spines, but covered 

 with extremely crowded imbricated scales. The Californian 

 variety is the most aberrant, being small, nearly round, and 

 never spiny. It might pass for a distinct species, were it not 

 that a few of Mr. Nuttall's specimens exactly belong to the 

 Mazatlan tjqje, while some few of the degenerated Mazatlan 

 specimens are closely allied to those from Monterey. — The 

 young shells which ^Alenke obtained from coral on Spondylus. 

 Chama, and Mures nigritus, and affiliated to the jS'ew Zealand 

 form, (so well marked that it received the same name from three 

 distinct sources), appears from the diagnosis to belong to this 

 species. 



C. aculeata belongs to the group of regularly spiral Cre- 

 pidulffi. It begins life as a smooth, glossy, light horn-coloured, 

 Velutina-shaped shell, with rapidly increasing whirls, and 

 a sunken apex. This is much larger than the nuclear part of 

 C. nivea, though the adult shell is much smaller ; being about 

 '025 across when it begins its second stage. C. nivea begins 

 with concentric ridges and goes on smooth, rapidly increasing, 

 but in a regular curve. C. aculeata makes a sudden expansion 

 when forming its deck, and then develops ridges as in the 

 form C. Lessonii. These soon become more or less undulated, 

 and then assume the state of vaulted spines, diff"ering in size 

 and arrangement, and in the period at which they commence. 

 At the same time various rays or spots of chesnuf colour 

 appear. Within, the growth of this species offers a well 

 marked contrast to that of C. nivea and its congeners. Instead 

 of forming a basal columella lip and then throwing up a deck 

 at a considerable angle, this shell makes the columella lip the 



