126 Trans. Acad. Sci. of St. Louis. 
Sinus broad, indefinitely limited anteriorly and feeble, sometimes barely 
traceable; shell usually thick in substance; genera entirely extinct and 
MOBtly EOCENE .....ceveccce -recceccccsecccsscses seve oo. FSEUDOTOMINI 
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- 
VEINS Mma co CooOdohO0G O50 ba0d bo adda pieisiatele aielceicantaorete --. MITROMORPHINI 
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 Zomella 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 
