BRACHTOPODA. 357 
])in()l)oliis iV) parvus.] 
Shell substance originally not very thick, apparently smooth and now replaC''d 
by a coarsely crystallized calcite, a feature common to many species of the Trimer- 
ellidse. 
Of this species we have but a single specimen from Minnesota, found by Mr. E. 
0. Ulrich, which is in an excellent state of preservation. The interior characters 
are shown in greater detail than appears to have been the case in the material from 
Wisconsin upon which Prof. Whitfield based the species, otherwise the example 
agrees with his description and illustrations. In size and form it is nearer to 
Dinoholus schmidti Davidson and King,* from the Lyckholmer Schicht at Kirna> 
Esthonia, a horizon nearly equivalent to the Trenton limestone of New York, 
than to Oholellina canadensis and 0. magnificiis Billings** from the Black River group 
of Canada, The latter, of which only the exterior is known, are also much larger 
species than D. ? parvus. 
The interior of D. ? parvus, as revealed in the Minnesota specimen, is highly 
instructive, since it appears to have characters both of the OboUdm and Trimerellida<. 
The structure of the cardinal region is similar to that of Oholella,^\\\\e the platform 
and its muscular scars are as in Dinoholus. The crescent so characteristic of Din- 
oholus is, however, absent in D. parvus. The diverging elevations on each side of 
the pedicle muscle scar in Oholella crassa, (the cardinal scars), are also present in 
D. ? parvus and occupy a position equivalent to the crescent of Dinoholus. It may 
be that here is indicated the line of development of the crescent from the cardinal 
scar of Oholella. The combination of these characters in D. ? parvus seems to require 
its separation from Dinoholus, but until the interior is known of the species referred 
by Billings to Oholellina it is better to leave it provisionally in the former genus. 
Lower Silurian species of Difioholus are also known from Esthonia {D. schmidti) and 
Brittany (D. hrimonti Rouaultf), but these have well-developed crescents. 
Formation and locality. — Middle Galena, Wykoff. 
Collector.—^. O. Ulrich. Type in the collection of C. Schuchert. 
♦Quart. Jour. Geol. Soc. London, vol. .\.\.\, p. 164, pi. .\ix, figs. 0, ISTl. 
"Canacl. Nat. Geol. vol. iii, p. 441, 1858; vol. vi, n, ser., p. 329, 187;,', 
tSee Davidson's paper In tlio Gool. Mas,, vol. vii, decade ii. p. 1140. ItWO. 
