316 PROCEEDINGS OF THE ACADEMY OF [1896. 



reliable in dealing with many species, especially in respect to geo- 

 graphic distribution. 



An American writer on natural history, Mr. C. J. Maynard, 

 some years ago begun the study of this genus, and to his earliest 

 publication on the subject we owe the first clear statement of some 

 facts of prime importance ; that the Cerions are excessively plastic, 

 and locally modified into a considerable number of species and sub- 

 species ; that the range of some of these forms is excessively limited ; 

 and that former authors had failed to discriminate many really dis- 

 tinct species, " lumping" them under a few old names; and finally, 

 that the aperture-armature, or " teeth " of the Cerions are variously 

 arranged, and furnish ground for the division of the genus into 

 several subgenera. Mr. Maynard, moreover, has discovered and 

 described a large number of most interesting species and varieties, 

 especially the Cayman Island group ; so that his work on this genus 

 has been an important one. However, in our opinion he has unduly 

 multiplied species and subspecies, basing them on characters we hold 

 to be too slight and inconstant, and his work is marred by inaccur- 

 acies of all kinds " too numerous to mention." 



Our object in preparing the present list has been primarily to place 

 before students a moderate estimate of the species of the group, 

 specific values being held neither in extremely narrow nor very 

 wide limits, but practically in conformity with the views represented 

 by the leading English and American conchological authors of to- 

 day. 



We have taken this occasion to place on record the results of a 

 careful study of a very large collection of shells of the genus, a 

 collection including numbers of shells which have been identified by 

 Bland, Swift, Pfeiffer, Dohrn, Gruner and others, as well as acces- 

 sions, considerable in the mass, from Messrs. H. D. Van Nostrand, 

 S. Raymond Roberts, W. H. Dall, C. J. Maynard and others. 



The soft anatomy of the Cerions is still but little known. Dr. 

 Leidy, the Cuvier of American Zoology, has given figures of the 

 the anatomy of C. incanum Binn.^ W. G. Binney has figured jaw 

 and teeth of the same species^ and C. J. Maynard has more recently 

 published figures of the jaws and soft anatomy of a species from 

 the Cayman Is.^ Leidy's figure unfortunately does not show the 

 various systems of organs separately, and it is difiicult to interpret 



^ Terrestrial Mollusks I, pi. xv, figs, ii-iv. 



2 Terr. Moll. V. 



^ Contributions to Science, Vol. I. 



