92 SEVENTH REPORT. 



GASTROSTEGES. 



In the matter of gastrosteges again there is a very remarkable difference. In brachy- 

 stoma there are 132 while 320 sirtalis average 151 with a range of 142-159. The average 

 number in sirtalis is thus 27 more than the number given for hrachystoma and the lowest 

 number here recorded for sirtalis is 10 more than brachystoma has. Brown gives the min- 

 imum of gastrosteges for sirtalis as 138, and even accepting his figures brachystoma has 

 five less. This is a wide divergence in an important character and certainly seems woithy 

 of being considered a specific difference. 



« 



BODY AND TAIL PER CENT. 



Brachystoma has a body per cent of total length of .752 and tail per cent of .248 while 

 212 male sirtalis show a body per cent of .769 and tail per cent of .231, a difference of nearly 

 two per cent. Moreover such a difference would be far more obvious to the eye than the 

 figures indicate. That it is .in marked contrast with sirtalis is shown by Brown's own 

 statement, that "the body of brachystoma is disproportionately short." 



COLOR. 



The imperfect condition of brachystoma and Cop's very incomplete description together 

 with the extraordinary variety of coloration in sirtalis and allied species make color dis- 

 tinctions of little value. Even if the coloration were the same these snakes are sharply 

 distinguished from each other by more important and constant characters. The con- 

 clusion follows from these facts that the specific characters of brachystoma separate it sharp- 

 ly from sirtalis. On the other hand as shown in the first half of this paper they are iden- 

 tical with those of butlerii and there can be no question but that brachystoma is a synonym 

 of butlerii and not of sirtalis. 



Olivet, Mich. 



THE DISTRIBUTION OF POLYGYRA IN MICHIGAN. 



BRYANT WALKER. 



This paper is based upon the records accumulated in the Census of 

 Michigan Mollusca compiled under the direction of the Academy, which 

 include all the collections both public and private known in the state, 

 and therefore represents very accurately the extent of our present 

 knowledge of the distribution of the genus in the state. 



The genus Polygyra is one of peculiar interest to students of American Conchology, as 

 it is pvu-ely North American in its range, not Ijeing known, either living or fossil, from South 

 America or any of the continents of the Old World. The region of the greatest differentia- 

 tion and therefore the probable centre of distribution is the southern portion of the Ap- 

 palachian Mountains. This accords with the geological fact that this region (in the way 

 of continuous emergence) is the oldest portion of the eastern part of the continent. 



Of the three subgenera now recognized in Polygyra, only two, Triodopsis and Stenotrema, 

 are found in Michigan, Polygyra s. s., with the exception of a single species found in south- 

 ern Indiana, not ranging north of the Ohio river. 



The species of Triodopsis found in Michigan, fall into two natural groups, which were 

 formerly considered of subgeneric value ; those with a more or less globose shell having the 

 aperture but slightly obstructed by labial and parietal teeth, of which P. albolabris is 

 the leading form, and those grouping around P. pnlliata, in which the aperture has a well 

 defined trilobed appearance, owing to greater development of the apertural teeth. These 

 two groups are separated in the following hst of all the species of the genus, which have 

 been recorded from the state. 



