172 



THE AMERICAN NATURALIST 



rS^OL. XL 



neiiry in the nervous system of the Rhipidoglossa and its absence 

 in the Docoglossa. Dialyneury, it will be remembered, is the 

 name applied to that form of nervous system in which the mantle 

 is innervated in part from the pleural and in part from the visceral 

 ganglia and in which these two systems of nerves are connected 

 by anastomosis. It may be added that the ventricle in Rhipi- 

 doglossa is usually traversed by the rectum although the Heli- 

 cini(Ue, which have but one auricle, are an exception to this rule. 



The Rhipidoglossa fall into two subsections: the Zygobranchia, 

 characterized by the possession of two gills and two auricles, which 

 are usually symmetrically disposed, although one gill may be 

 smaller than the other; and the Azygobranchia with but one gill 

 and ordinarily with two auricles, one of which is more or less 

 rudimentary or even absent altogether. The Docoglossa have a 

 single gill (ctenidium) or none at all, a single auricle with no trace 

 of a second,* and a heart whose ventricle is never traversed by the 

 rectum. It is thus evident that the three divisions of Scutibran- 

 chiata form in many respects a continuous anatomical series 

 whose members, whatever the view as to their phylogenetic rela- 

 tionship, may often be profitably compared. 



The Docoglossa 2 include three families: the T.epctidic. Acmiei- 

 da^, and Patellid«. The Lcpctida' is a small family wIk.sc mem- 

 bers inhabit water of considerable depth. Of its fourteen sjx^eies 

 and varieties only one is n-conled as living at low-water mark; 

 the others have been dredgeil at depths varving from ten fathoms 

 (five fathoms in one instance) to thirteen Innidred ninety-five 

 fathoms. While in certain respt>ets, as in the shell and radula, 

 they exhibit relationship to the Rhij)idoglossa, in others they are 

 modified in correlation with their environment so that they appear 

 to be, as maintained by Dall, less typical Docoglossa than are the 

 others. Thv family contains three genera: Lepeta, (including 



