GASTROPODA. 1009 
Plethospira.] 
Fie. 7.—a, Plethospira cassina Whitfieid sp , Calciferous formation, Fort Cassin, Vermont. Some- 
what restored view of a good specimen belonging to the U. 8S. National Museum. The last whorl retains 
much of the shell, and shows the characters of the band and surface markings. }, c,d, Seelya ventricosa 
Ulrich, n. sp., Calciferous formation, Fort Cassin, Vermont. 6, Cast of the interior of a rather small 
specimen, collected by Prof. H. M. Seely and presented to one of the authors. The ridge which forms the 
lower margin of the band is not prominent enough in the figure. c¢, Surface of the shell of a species of 
Seelya from Fort Cassin, x2, taken from anincomplete specimen belonging to the U.S. National Museum, 
showing the band and surface markings. d, small portion of the same, x10, showing the grano-punctate 
character of the outer surface. This specimen probably belongs to Seelya difficilis Whitfield sp. Another 
specimen of S. ventricosa in the National Museum is much larger than }, being 57 mm. high and 40 mm. 
wide. The last whorl descends very much as in a, and is marked by five ridges above and five below the 
band. 
Plethospira probably will not prove to be a large genus, yet, as it represents an 
easily recognized type of shells that moreover continued through a long period of 
time, it deserves and probably will receive recognition from systematists. P. cassina 
Whitfield sp. is a fine species from the Calciferous rocks of the Lake Champlain 
region. A second species from that region was somewhat doubtfully identified by 
Whitfield with Billings’ Murchisonia arenaria. For some unaccountable reason 
Whitfield places both of these Calciferous species under Holopea. A third species, 
evidently very closely related to the preceding, was called Murchisonia hyale by 
Billings, who gives it as from “the Chazy or perhaps the Black River limestone” of 
Canada East. The same author has described other Lower Silurian shells that may 
belong here, but, as we have not yet seen his original types, we prefer to postpone 
their removal from the genera to which he assigned them. Lindstrém’s Pleuroto- 
maria valida, an Upper Silurian species, has a very thick shell and rather too high a 
spire, but otherwise seems to agree very well with Plethospira. Regarding the 
following two species, perhaps they are not very good examples of Plethospira, still, 
it would be difficult to pick out any important differences; at any rate, we cannot 
suggest a more fitting disposal of them at present. 
