THE NAUTILUS. 55 



acter, namely the beautiful fine granulation which covers even the 

 ribs and shows a markedly spiral arrangement. This is quite differ- 

 ent from the spaced spiral incised lines of perigrapta, which while 

 slightly waved, cut the growth strias so regularly as to produce a can- 

 cellated effect, the intervening surface being moreover always marked 

 with excessively tine vertical scratches. A compound lens reveals 

 incised spirals in typical appressa also, but exceedingly minute and 

 rather irregular or punctate. Finally, in sctdptior the whorls are 

 generally narrower and more convex than in either of the other 

 forms. In the tray-full examined there was but a single departure 

 from the normal, and that was a tridentata! 



The types are deposited with the Academy of Natural Sciences, 

 Philadelphia. 



SUPPLEMENTAL NOTE ON THE MOLLUSCA ASSOCIATED WITH THE 

 MASTODON IN BEKR1EN COUNTY, MICHIGAN. 



BY BRYANT WALKER. 



The material which formed the basis of Mr. W. Hilles Smith's in- 

 teresting paper in the July Nautilus, is the same covered by my ar- 

 ticle in the Nautilus for March, 1898 (Vol. XL, p. 121). The 

 undetermined Pisidia there mentioned have been submitted to Dr. 

 Sterki, who, with his usual good nature, has determined them as 

 follows : 



1. Pisidium pauperculum St. 



2. Pisidium sp. near " abditum" Distinct. 



3. Ptsidium sp. "near abditum." 



4. Ptsidium i-operi St. 



o. PisidiiiDi medianum St. 



6. Pisidium sp. " Resembles most vesieulare." 



7. Pisidium ventricosum Prime. 



8. Pisidium milium Held. 



In this connection it is interesting to note that these forms are 

 identical with those from the marl deposits in Tuscola County, which 

 were examined by Dr. Sterki at the same time. This not only goes 

 to prove that these shells were contemporaneous with the mastodon, 

 but also that the post-pleistocene fauna was substantially the same 

 throughout the lower peninsula of Michigan. 



