274 The American Geologist. isovember, i892 
upon general resemblances rather than upon unimportant, variant 
characters arising from diverse conditions of environment imposed 
by a more or less extensive geographic distribution. Thus, in 
choosing for classification purposes the characters of any group, 
it is evident that only those features exhibiting the least tendency 
to modification are available. Even the most constant structures 
appear to lose much of their stability at some period during the 
existence of the group — whether specific, generic or family, while 
other characters, more or less varient in the early stages of 
development, later become less liable to change. At some time 
or other these features blend and thus arise the transitional forms. 
It may be assumed then, that in many groups of the same 
genetic origin some varieties will present features that have 
remained for a long time practically unmodified; while others ex- 
hibit the same characters in a highly specialized, but ever chang- 
ing, condition. And it is of great interest to note in this 
connection that the latter — those having greatly exaggerated feat- 
ures — are the fgrms whose existence is of comparatively short 
duration; and that with these intensified structures the develop- 
ment is rather rapid, while their culmination is soon followed by 
a great diminution of the group's vitality, or more commonly its 
extinction. 
Under Conrads generic name upwards of three hundred species 
of gasteropods have been proposed. In such a group of shells 
having so few salient characters for classification and a great range 
of variation, it is not hard to foresee some of the difficulties to be 
encountered in attempting to arrange satisfactorily the many dif- 
ferent forms. The placing of Platyceras, Orthonychia, etc., as 
sub-genera under Capulus, as has been done by Zittel* and others, 
manifestly does not meet the requirements, especially as regards 
the American members. Moreover, the group has been made to 
embrace a great variety of species, some of which are clearly not 
at all closely related genetically. Of these a few forms have been 
referred lately to the places to which they more properly belong. 
But there are still a considerable number of these shells which are 
evidently not members of the group, yet whose generic affinities 
cannot be determined at present, with exactness. 
In the recently issued synopsis! of the Calyptrsidit occurring 
*Handb. der Pal., ii Band, p. 216. 
tProc. Acad. Nat. Sci., Phila., pp. 150-181. (1890.) 
