Casey — Notes on the Pleurotomidae. 165 
occur, for the most part, in middle and upper Tertiary strata. 
The numerous genera cluster about certain apparently isolated 
type forms such as Clathurella, Glyphostoma, Cythara, 
Mangelia, Daphnella and Raphitoma, which differ so greatly 
among themselves as to suggest the propriety of subtribal 
division, but after long and patient study of rather large ma- 
terial I have been unable to devise a system of characters to 
serve for the definition of these subtribal groups. Cythara, 
in its typical forms, is arather heavy shell, sometimes remind- 
ing us of the Conidae, having a long oblique linear aperture 
and well developed labial plicae, but other forms occur in 
which the linear aperture shortens by degrees and becomes 
devoid of folds, giving us the conditions observed in Mangelia. 
Others, having a short but gradually more oval and plicifer- 
ous aperture, merge from Mangelia into Gilyphostoma and 
these into Clathurella, which is a larger and more obese form 
with thinner shell substance, and Clathurella again into Raphi- 
toma and Bellardiella, which usually have a thin non-plicate 
outer lip, and Daphnella, with very thin fragile shell walls 
and absence of the true ribbing so universal in the remainder 
of the tribe. The short or obsolete beak of Daphnella be- 
comes elongate in Teres and still longer in Pleurotomella, and, 
in Hucyclotoma Boettg., we have, as frequently occurs, a 
remarkable special structure of the shell as well as an embryo 
differing radically from that of Daphnella, to which it is allied 
by the absence of true ribbing. 
These various transitions are made through more or less 
small but abrupt differentials, indicating intermediate generic 
a subgeneric groups, but the genera are so numerous in pro- 
portion to the known species, that one may well hesitate to 
define them, although it should be stated that if these inter- 
mediate stages are not characterized as genera it will be im- 
possible to set any definite limits to the principal genera 
named above. When fuller series of species shall have been 
collected throughout the world and scientific workers become 
more discriminating than at present, generic names will 
certainly have to be given a very large number of these re- 
markable type forms, but at the present time little or no use- 
