596 LAWS OF ORGANIC DEVELOPMENT. 
LICIDA. 
1. Turns of spire very few; tae poser shell hry ag with thin aye oo’ gee: 
2. Turns few, but more; see 
3. Turns still more numerous; sais as alk please, ee tes Sino. E inti 
4. As No. 3, but lip thickened inside, ate gromia. 
5. Coiled; umbilicus closed; lip tildkenéa ER wå Jout, rts gadha tid pidhin 
6. Same, with a parietal tooth f AT ies Mesodon. 
7. Same, — starter mi two futero lip ton NOA ia O ier rae __Tsognomostoma. 
mencing at No.4. All with op bili 
5. As No. 4, =f hip ickened in in and.out,. 5. ei $ roe 
6. Same as No. 5, but with parietal tooth, . . e . 1. 1. + e ss «+s Patio 
“7. Same, with Sani ‘esis and lip teeth, . PS EP a, 2 
The successional relation of these genera may be represented in 
such a diagram as this :— 
Umbilicus opened. Umbilicus closed. 
* . * 
* 
* 
xk 
* 
TCT oto) 
In the history of the growth of the genera Eioguonnoetonen and 
Triodopsis, 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 as in Hygromia. Then 
the umbilicus may close as in Tachea, or (in Triodopsis) remain 
open as in Arionta. In either case a tooth is soon added on the 
body whorl (Polymita, Mesodon), and finally, the full maturity of 
the shell is seen in the added width of the inside of the lip-margin. 
How many of the stages of the genera Triodopsis and Mesodon 
are identical with the genera of the series which represent them 
I leave to more thorough conchologists, but that some now exhibit 
and all have presented illustrations of the relation of exact paral- 
lelism I cannot doubt. 
Example 1.— An abundant race of the American deer, Cariacus 
Virginianus, exists in the Adirondack region of New York, in 
which the development of the antlers never progresses bey ond 
the spike stage of the second year. Therefore, some individuals 
of this species belong to Cariacus and some to Subulo. 
Example 2.— A large part of the individuals of the common 
snail, Mesodon albolabris, never develop the tooth of the body- 
whorl, characteristic of the genus whose definition has to be mod- 
ified to retain them. 
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