38 THE NAUTILUS. 



it does occur in any species, it is, apparently, always present 

 to some degree. As a specific character, therefore, the presence 

 or absence of eversion can be relied upon, but the amount of 

 eversion, when present, would seem to be an individual, rather 

 than a racial or specific peculiarity. 



The amount of eversion is usually coincident with the height 

 and acuteness of the spire. In species having a comparatively 

 high, acute spire, the apical whorls are more closely coiled and 

 the apical whorl, itself, is very convex and separated from the 

 succeeding whorl by a deep, constrictive suture. In species 

 with a blunter apex the apical whorl is less convex, being well 

 rounded above, but less constricted below b}^ the suture. In 

 all of these forms, the apex itself, when viewed laterally, is not 

 acutely pointed. It is more or less obliquely flattened on top 

 and the tip, or what would be the tip if it were everted, is 

 below the level of the surrounding whorl and often is, as it 

 were, immersed in the axis. 



In species with flat or planorboid apices, there is no eversion 

 of the apical whorl, which is not constricted by the suture, but 

 is flatly rounded above and separated from the succeeding whorl 

 by a well defined, but vertical suture. 



In nearly all of the species that have been examined, there is 

 a well defined apical sculpture. In Somatogyrus, apparently, 

 the embryonic shell has only a single whorl and the apical sculp- 

 ture, when present, is confined to that whorl. It is wholly 

 epidermal in character. The epidermis being very thin and 

 delicate, it is extremely subject to erosion and frequently has 

 entirely disappeared in specimens, which, so far as the shape 

 of the whorl is concerned, are substantially uneroded. For this 

 reason, in several of the described species none of the specimens 

 on hand were sufficiently perfect to afford any information as 

 to their apical sculpture and in others the apices of all of the 

 specimens were entirely eroded. 



The apical sculpture, when present, is exceedingly fine and 

 delicate and, even in perfect specimens, usually requires a lens 

 of 100 diameters to develop it fully. It is, apparently, quite 

 constant in its character in each species and, when different in 

 detail, would seem to be a good specific character. 



