824. THE PALEONTOLOGY OF MINNESOTA. 
(Tryblidium. 
Genus TRYBLIDIUM, Lindstrém. 
Tryblidium, LiNpDsTR6M, 1880, Fragmenta Silurica, p. 15; 1884, Silurian Gastropoda of Gotland, p. 52. 
Tryblidium (part.), WHITEAVES, 1884, Paleozoic Fossils, vol. iii, p. 830. WHITFIELD, 1888, Bull. Amer. 
Mus. Nat. Hist., vol. i, p. 303. 
Metoptoma (part.) of BILLINGS and other authors. 
For generic diagnosis see page 821. 
Our description of this genus does not agree in all respects with that given by 
Lindstrém. This is partly because we believe his to be incorrect in several partic- 
ulars, and partly because we have seen fit to leave out as unimportant one or two 
features, and to mention the style of surface markings. Of the three species 
described by Lindstrém in his last work (op. cit.), T. reticulatum, T. unguis and 
T.2 radiatum, we would strike out the first and the last, while to the second we 
would assign the rank of type of the genus. This proceeding is not strictly in 
accordance with usage, since when, as in this case, no type is specified, it is custom- 
ary to regard as such the first species following the generic description. The rule 
is both a good and a necessary one, yet there are cases, and we believe this is one of 
them, where it is best not to follow it. 
The species reticulatum and unguwis, of which we have through the kindness of 
Dr. Lindstrém good examples before us, represents according to our views two 
generically distinct types, of which the first is limited to one or possibly two 
species,* while the second is recognized in numerous Lower and Upper Silurian 
species. Herein lies the reason for the course here pursued with respect to the type 
of the genus, for, should our view of the generic distinctions of the two species 
available as types prevail, the greater justice and credit would accrue to the learned 
author of the genus if his name is adopted for the more abundant group of the two. 
As to the third species, 7. ? radiatum, we refer it with much confidence to our new 
genus Helcionopsis. 
T. unguis and all the other species which we leave under Tryblidium have a 
concentrically striated thin shell composed, as far as known, of thin glossy lamelle 
which are never porous. 7’. reticulatum, on the contrary, has a thick shell, especially 
at the edges, the external layer is minutely porous, and the surface strongly marked 
by salient oblique concentric lamin, which in the anterior part cross each other, 
producing an “engine turning” style of network. Lindstrém is inclined to regard 
the porous character of the outer layer as produced by some parasitic organism. 
Such an explanation of the origin of the pores would no doubt be a good one if they 
occurred only occasionally, but considering the fact that they are always present in 
the Gotland specimens, and what is more, that they seem to be uniformly distributed 
over the whole exterior stratum, we may well question its sufficiency. The view 
* Lindstrém mentions a Lower Silurian species from Esthonia which he says is closely related to T. reticulatum. 
