OF CONCIIOLOGY. 235 



the difference of the shell alone, however, is of minor inipor- 

 tance, and without other characters affords no safe ground for 

 classification, and is utterly unreliable in determining aflBnities. 



In one case only am I in doubt as to the status of a name 

 which I have here provisionally adopted. I refer to the sub- 

 genus JTelciom'scus, which may require eventually to be merged 

 in the genus Helcion. The dentition differs essentially from that 

 of Patina, and the shells of both are widely removed from that 

 of the typical Helcion pectinatus, of the Mediterranean, the only 

 species of th&t genus as far as known ; yet one or the other may 

 prove eventually congeneric. 



Unfortunately the dentition of the type o^ Helcion is unknown. 

 When it shall be worked out the question can be satisfactorily 

 settled. Meanwhile, rather than lose sight of the characters 

 which are known, in Patina and Helcinnella^ I prefer to retain 

 the three subgroups until we can unite or definitely separate 

 them by actual knowledge of their relations. 



I find, by wide comparison of specimens, that the forms indi- 

 cated by the names Cjiinbida, Scutelladra, and even Olana, are 

 due to habitat rather than specific characters, and merge imper- 

 ceptibly one into another, in different and even in the same spe- 

 cies ; hence I feel justified in referring them to Patella as syno- 

 nyms. ScutelUna, again, dt)ubtless contains species which should 

 be referred to several groups already described, and very possi- 

 bly some which may stand as distinct; yet the animal of the 

 only species which has been described does not seem to differ at 

 all from Acmoia, as far as we know its characters, and hence I 

 prefer to omit the name in the series ; giving it, with its own 

 exclusive synonyms under it, as a probable synonym o1 Acma^a, 

 Esch., in part, and for the remainder suspending judgment until 

 we obtain more information. 



With regard to the synonymy, no opportunity has been omit- 

 ted of consulting the original descriptions, rather than the unre- 

 liable and often erroneous references of authors who have not 

 taken the trouble of verifying their opinions. It is believed to 

 be essentially accurate, and only liable to change from a more 

 thorough examination of the characters of the type of Helcion. 



Trusting that the result of my labor may be to throw some 

 light upon an interesting and very complex subject, I would :isk 

 the cooperation of other students in extending these researches 

 still further, and in correcting any of the almost inevitable 

 errors which may be found in this as well as almost all similar 

 work. 



