126 Trans. Acad. Set. of St. Louis. 



Sinus broad, indefinitely limited anteriorly and feeble, sometimes barely 

 traceable; shell usually thick in substance; genera entirely extinct and 

 mostly Eocene Pseudotomini 



6 — Shell very small, slender, thick in substance and with short aperture, 

 surface strongly sculptured, ribbed or cancellate Donovaniini 



7 — Anal sinus small but well defined, usually deep and distinct, adjacent to 

 or very near the suture ; shell varying greatly in size and thickness of sub- 

 stance, mostly living, the genera and species numerous.. . .Daphnellini 



Anal sinus very broad and ill-defined, with its deepest part on the obtuse per- 

 iphery near the middle of the whorls; shell minute in size and fragile in 

 substance; species few in number; Eocene to present time.. ..Taranini 



Anal sinus obsolete; shell small in size, mitriform, with long narrow aper- 

 ture, thick in substance; species few in number, recent in develop- 

 ment MlTROJIORPHINI 



These tribes are not all rigorously isolated or strongly de- 

 limited and there are several generic types before me which, 

 from general appearance, may prove to be annectant or 

 doubtfully referable to any of them, but they serve very 

 well in a broad way and there is very seldom any doubt as to 

 the proper reference of a species by its general facies alone. 

 The genus Tomella may perhaps best serve as an illustration 

 of these aberrant forms. This genus, while evidently bearing 

 a phylogenetic relationship with Perrona, as shown by the 

 embryo and nepionic whorls, differs so greatly in other charac- 

 ters that it not only cannot be regarded as at present closely 

 allied to Perrona, but must be considered one of the most 

 remarkable genera of the entire family. While evidently a 

 member of the tribe Pleurotomini, it departs from its most 

 significant characteristic in possessing a large deposit of callus 

 at the posterior limit of the aperture, though, upon closer 

 observation, it can be perceived that this callus in not quite 

 homologous with the posterior tumid callus of the Clavini and 

 is probably formed in a different way. The chief peculiarity 

 of Tomella resides in the form and anterior position of the 

 anal sinus, and in the fact that this sinus is wholly covered 

 and concealed from view by the successive growth of the 

 shell, so that it does not appear upon the spire whorls — a 

 character which isolates it completely in the family. 



In making use of the characters before referred to as legit- 

 imate criteria for the definition of genera, a large amount of 

 comparative study and latitude of interpretation will be 



