1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 811: 



anatomical memoir since Isenkrahe published an incomplete 

 account of their structure in 1867, and it is high time that this 

 gap in our knowledge of Gastropodan anatom}^ should be filled 

 up. If comparative anatomists have given but small heed to the 

 Neritacea, the conchologists and systematists have done their full 

 share of work on the group, and the works of Martens (27) and 

 Pilsbry and Tryon (40) give a most complete account from a 

 taxonomical point of view. But, as is often the case when classi- 

 fication is founded on external characters only, such as the shell and 

 the operculum, a more complete study shows that it rests on 

 insecure foundations ; and while I am unable to do more than 

 criticize the genera Nerita and Neritina as usually defined, I shall 

 bring forward evidence which will, I hope, induce authors more 

 conveniently situated for the study of these forms than I am to 

 undertake a revision of the family Neritidpe, based upon anatomical 

 characters. 



The name Neritacea — the group has not been raised to the 

 rank of a suborder or even of a tribe or section — w^as used b}' 

 Lamarck as a collective designation for the recent families of 

 Neritidje, Is eritinidte, and Helicinidte. To these have been added 

 the Neritopsidae, Titiscaniidse, Scutellinidie, Hydrocenidte, and 

 Proserpinidee, and the fossil families Maclureidge and IS'aticopsidas. 



It is not part of my present intention to criticize the recent 

 and extinct families and genera that have been founded by 

 conchologists, nor to discuss the probable relationships of the 

 palfeozoic forms which, like the genus Deshayesia, have been held 

 to occupy a position intermediate between the I^eritida? and 

 Naticidae (see Petho (33), who refers it to the latter family). 

 But as it will appear in the latter part of this paj)er that the 

 result of my anatomical investigations is to show that the 

 Neritacea retain some primitive characters, and in so far as they 

 are specialized do not show any approximation to the Pectini- 

 branchia, but are contrariwise modified in a special direction 

 which culminates in the terrestrial HelicinidEe, cfec, it is of 

 interest to consider how far the group may be regarded as of 

 undoubted geological antiquity. The family Neritid^ is of 

 respectable antiquity. The genus Nerita is I'epresented by the 

 subgenus Lissoch'dus Petho in the Triassic and Jui'assic, and b}- 

 the subgeniis Otostoma d'Archiac in the Cretaceous of Europe,. 

 Algiers, and Asia Minor. The genus Deshayesia, which is con- 

 sidered by some conchologists to " present a very remarkable 

 combination of the characters of Keriia and Katica and appears 

 to establish a passage between those genera" (Pilsbry and 

 Tryon, vol. x. p. 5), is from the Eocene and Miocene of the Paris 

 and Bordeaux basins, and if it is really a Iseritid, its Naticid 

 characters must be due to convergence and must not be taken as 

 indicating a passage between the Aspidobranchiate and Pectini- 

 branchiate Gastropods, for such a passage must have been effected 

 long before the Tertiary period. 



The genus Neritina, if indeed it is as distinct frcm Nerita as. 



