1871.] 230 
[Cope. 
The successional relation of these genera may be represented in such a 
diagram as this: 
Umbilicus open. Umbilicus closed. 
* * 
* * 
mw ROD A 
KF * 
In the history of the growth of the genera Isognomostoma and Trio- 
dopsis, the extreme forms of the two series, it is well known that at first 
the coils of the shell are extremely few, as in Binneya; and that like it, 
it is very thin and with a delicately thin edge; that the turns increase 
successively in number, as in Vitrina and Hyalina, and that finally the 
lip thickens asin Hygromia. Then the umbilicus may close asin Tachea, 
or (in Triodopsis) remain open asin Arionta. In either casea tooth is soon 
added on the body whorl (Polymita, Mesodon), and finally, the full ma- 
turity of the shell is seen in the added teeth of the inside of the lip margin. 
How many of the stages of the genera Z7todopsis and Mesodon are identi- 
cal with the genera of the series which represent them, I leave to more 
thorough conchologists, but that some now exhibit and all have once 
presented illustrations of the relation of exact parallelism, I cannot doubt. 
Heample 1.—An abundant race of the American deer, Cariacus virgi- 
nianus, exists in the Adirondack region of New York, in which the de- 
velopment of the antlers never progresses beyond the spike stage of the 
second year. Therefore, some individuals of this species belong to Cari- 
acus and some to Subulo. 
Heample 2.—A. large part of the individuals of the common snail, MJeso- 
don albolabris, never develop the tooth of the body-whorl, characteristic 
of the genus whose definition has to be modified to retain them. 
Heample 3.—Many individuals of Triodopsis tridentata from eastern 
North Carolina occur without the lip-teeth, characteristic of the genus 
Triodopsis. Hence these specimens, though of common origin with others 
of the species, must be referred to another genus. 
Hxample 4.—Structural characters are known in many, if not all, species 
which are said to be ‘‘inconstant,’’ being present or absent indifferently, 
thus being useless for definition. They may be rudimental when present 
or considerably developed. The presence or absence of wings in some 
species of insects may be cited; also the presence of generic characters 
in the male sex of many Coleoptera and their absence in the females. 
‘The characters of males, females, workers and soldiers in bees and ants 
may be added. All these facts belong to the same category as those cited 
among deer and mollusks and have a similar explanation. 
Hzxample 5.-—It does not seem to be the law in “‘retardation’’ that par- 
allelisms exhibited by the series in its rise to its highest point of develop- 
ment should retrace the steps by which it attained it, and that ‘‘ exact 
A. P. S.—VOL. XII.—2D. 
