THE NAUTILUS. 99 



naturalists, which have come under my notice liavc all appeared to 

 me to be referable to C fornieata (L) Lam., or to the variety of 

 C. convexa Say which has grown upon a broad, not very convex, or 

 perhaps a partly concave surface. A fine series of shells from ('ape 

 May, submitted to me by Mr. Ford as C. glauca come under the 

 latter category. They are distinct from C. foriilcata, but thoy do 

 not show the cliaracters called for by Say's description, which in 

 itself presents no features to distinguish the shell he describes from 

 the young and spotted form of C. fornieata, to which I have no 

 doubt it belonged. Such shells are in the National ^luseum labeled 

 C. glauca by Stimpson. It may not be known to all our concliolo- 

 gists that the extremely high and usually dark-colored forms of 

 Crepidnla, such as C. convexa (typical) and C. adunca of California, 

 derive their peculiar appearance from roosting on some gasteropod, 

 and that wider and less elevated forms of the same species can 

 almost always be found, if not in the same locality, at least in a 

 geographical series of wide range. The difficulties, in the way of a 

 satisfactory determination of such semi-parasitic forms as the Calyp- 

 trceidce are very great, and increase in my experience with the 

 amount of material subjected to study. That there may be a distinct 

 form entitled to be called C. glauca I do not wish ]>ositively to deny, 

 but I must confess that I have never seen one and therefore have 

 irrave doubts of its existence. 



A LIST OF THE MOLLUSCA OF COLORADO. 



(Compiled for the Colorado Biological Association.) 



BY T. D. A. COCKERELL. 



In the '• Journal of Conchology," vol. vi, pp. 60-65, I gave some 

 preliminary remarks on the Mollusca of Colorado, stating that from 

 the lack of records and other reasons, a complete list could not then 

 be prepared. Since then a considerable amount of material and 

 information has been gathered, and while much remains unknown, 

 there is sufficient ascertained to seem to warrant the presentation of 

 a faunal list. In this list the localities are not all given in detail, as 



