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, Gythara, 

 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. Gythara, 

 in its typical forms, is a rather 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 Glyphostoma and 

 these into Clathurella, which is a larger and more obese form 

 with thinner shell substance, and Clathurella again into Raphi- 

 toma and Bellarcliella, 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 Eucyclotoma 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- 



