OF CONCHOLOGY. 281 



peculiar persistency of immaturity, when compared with other 

 groups of Gasteropods. The embryonic shell is dish-shaped, ac- 

 cording to Carpenter and Fischer, and this form is persis- 

 tent in the adult. The dentition is characterized by a want 

 of solidification of the chitine, especially in the Patellidce, 

 «nd the teeth of some forms are represented by mere unde- 

 veloped bosses in others, and even these are absent in a third 

 series. In no group are the fundamental outlines of the bases 

 of the teeth so difficult to make out or represent by figures. 

 Their variation, within certain limits, is noticeable, and a ten- 

 dency to melt one into another, so to speak, may frequently be 

 observed. The teeth can be clearly separated into three series, 

 median or rhachidian, lateral and uncinal. The lateral teeth 

 are distinguished from the uncini by being situated upon the 

 «rea, while the uncini are invariably upon i\\Q pleurce. 



The order is obviously separable by the dentition into two 

 groups, viz., those with lateral teeth and those without them. 

 The first is almost invariably destitute of a rhachidian tooth ; in 

 one form {Acmcea as restricted) we find no uncini or median 

 teeth; in another i^Ancistromesus, n. g.) we find all of the series 

 represented ; but, throughout the group the lateral teeth are 

 persistent and three in number. The second group is furnished 

 with a large and prominent rhachidian tooth and several uncini. 



These differences are coordinated by others in the soft parts. 

 Those possessing lateral teeth are also furnished with eyes and 

 branchiae. Those without laterals are also destitute of the above 

 mentioned organs. 



The latter comprise the family Lepetidce, and will take the 

 subordinal name of Abranchiata. 



As the examinations detailed in this paper show that the sub- 

 orders Cyclo- and Cervico-branchiata are not separable from each 

 other by any characters whatever, being combined in some in- 

 stances in a single individual ; and as the terms above mentioned 

 convey an error, inter se, and have also been applied to other 

 groups, I shall propose for the group formed by their consolida- 

 tion the subordinal name of Proteobranchiata. 



The Proteo-branchiata are divisible into two groups, by ana- 

 tomical characters as well as the dentition ; though the sheila 

 afford no high diagnostic characters. These comprise, 1st, those 

 possessing a distinct cervical branchia and without a median 

 tooth, and not more than two uncini. 2d, those without a cer- 

 vical branchia, with or without a median tooth, and with three 

 uncini on each side. 



The first group will form the restricted family Acmceidce, and 

 the second the Patellidce. Both groups are furnished with three 



