ATAPHRUS. 347 



of our Inferior Oolite this feature of a columellar furrow is greatly subordinate 

 to the columellar callosity which generally culminates in a sort of denticle — 

 the small tubercle of Gemmellaro's genus Plocostylus. This feature was especially 

 recognised by d'Orbigny in his description of Trochus Belus and Trochus Acmon, 

 the former of which was regarded by Gabb as typical of his genus Ataphrus. 



On the whole, it would seem that Gemmellaro's genus Plocostylus^ slightly 

 modified, would embrace the depressed forms of the smooth " Monodontas " more 

 correctly than Ataphrus. But it is desirable to place the trochiform as well as the 

 turbinate "Monodontas" under one genus. This view was adopted by Messrs. 

 Hudleston and Wilson in their ' Catalogue of British Jurassic Gasteropoda.' 

 Notwithstanding certain difficulties, to which allusion has been made, a similar 

 arrangement is adopted in this Monograph. The following diagnosis, although 

 somewhat different from that of Gabb, relates more especially to the fossils of the 

 Inferior Oolite described below. If this diagnosis is inapplicable to Ataphrus, we 

 must look for another generic name. 



Shell thick, smooth, and generally small, turbinate or trochiform, imperforate; 

 spire obtuse and often much depressed. Body-whorl relatively large, base more or less 

 flattened, aperture subcircular and restricted. The columellar lip is very short and 

 much incrusted, so that the thickening usually assumes the form of a small blunt denticle 

 towards the extremity of the columella. Growth-lines extremely fine and close. 



The above diagnosis possesses many of the characteristics of Plocostylus, but 

 is more comprehensive, since it includes trochiform species, such as Ataphrus Acts. 

 In fact, as far as the Inferior Oolite is concerned, it comprises a regular ascending 

 sequence from ovulate to trochoidal forms. The question of texture is important in 

 denning the genus. In certain matrices these shells present a glabrous appearance, 

 due to the extreme fineness of the growth-lines, which very often are invisible. The 

 glabrous surface, the blunt and obtuse character of the apical region, the full 

 whorls, and the closely fitting suture are distinctive features, quite as much as the 

 columellar callosity, or the furrow with which it is associated. 



In assigning specific names I have endeavoured in nearly all cases, even at the 

 risk of stretching a point, to make use of those already in existence, and of these 

 there is a plentiful supply. It need scarcely be said that forms intermediate to 

 those named also present themselves, and are not easy to allocate. There does not 

 seem to be any special relation between the species and the horizon. 



