MONOGRAPH OF TUB GENUS STROPHIA. 
15 
DIMENSIONS. 
Size of type 1.05 by .45. Largest specimen 1.25 by .50 ; smallest 
1.00 by. 40. Longest specimen, 1.25 ; shortest, 1.00. Greatest diam- 
eter, .50 ; smallest .40. 
OBSERVATIONS. 
Variation consists in specimens being either lighter or darker than 
the type, in other words, the markings are more or less abundant, thus 
covering more or less of the surface of. the shell. There is one distinct 
form as given below. 
Form No. 1. Larger than the type, 1.25 by .50, otherwise simi- 
lar. 
This well marked sub-species differs from typical S. marmorata 
in the absence of striations, in the more cylindrical form of the shell, in 
the singularly diverging double teeth, and less converging margin. It 
is with considerable hesitation that I do not admit this form to full 
specific rank, but as I find some individuals which show evidence of 
intergradation, and as I found them in the same tray with the spec- 
imens of S.marmorata, described above, and consequently labeled as 
coming from the same locality (Carbo Cruz, Cuba. ) I have thought it 
best for the present to consider them as sub-species only. 
The difference in the teeth given above, in two such closely allied 
forms as S. marmorata and S. m. polita is worthy of note, as it exactly 
coincides with the theory, which I have had oc- 
casion to mention that the teeth of the members 
of this genus are vital characters, for as the an- 
imals have no muscles which grow to the shell 
anywhere, they depend largely upon the teeth 
as a means for holding themselves on the shell, 
therefore, with any change of environment which 
would tend to alter the weight of the shell would 
also tend to change the form of teeth. In the 
accompanying cut fig. 2 A. I give a figure of 
the teeth of S. marmorata and at ib. B, the same of S. m. polita. 
There is a group of land shells which occur in the island of Mau- 
ritius, now included in the family Pupidae, which are closely allied to 
Strophia, and which possess similar teeth. In these shells the evolution 
of the teeth, both upper and lower, is clearly indicated. At first it is 
evident that the margin about the aperture was projected forward all 
around, when the animal for some reason lost its attachment for the 
shell, and required a tooth, then a portion of the margin, below the 
mouth, was bent inward, afterwards becoming dedatched to form a 
tooth ; afterwards a second was formed in a similar way. 
Fig, 2. 
A, Teeth of Strophia. 
B, Teeth of S. m. polita. 
Sectional view of shells. 
