128 THE PALEONTOLOGY OF MINNESOTA. 
[Rhinidictya. 
is usual in the genus), are oblique and inclosed by a strongly elevated peristome, 
highest at the posterior side. They manifest further a tendency to arrangement in 
transverse or diagonal rows. The result is quite unlike what is to be expected in 
Rhinidictya, and reminds one more of certain species of Cystodictya. Thin sections, 
however, demonstrate that this is merely a case of superficial resemblance and not 
of true relationship. On the contrary these prove, as is already clearly enough 
shown at the growing extremities of the branches, that we are dealing with a true 
Rhinidictya with affinities to Rk. mutabilis too close to admit of even specific distine- 
tion. Indeed, it is not improbable that the variety represents merely an unusual 
condition of senility. Still, the interior, as exhibited in the sections at hand, has one 
feature that may be accepted as corroborating my present estimation of the form. 
* Plate VI, fig. 2, represents a portion of a tangential section showing, besides one 
of the solid axillary macule, that the minute interstitial tubuli are exceedingly 
numerous, there being often three longitudinal rows between adjoining zocecia. 
Figure 3 of the same plate presents a portion of a vertical section of the same speci- 
men. This compares very nearly with figs.5 and 12 (pl. VI) prepared from old exam- 
ples of the typical form. The absence of horizontal lines in the lower part of the 
walls may be the result of imperfect preservation. 
This species, especially in its typical form, is to be regarded as closely allied to 
R. nicholsoni Ulrich (Jour. Cin. Soc. Nat. Hist., vol. v. p. 170, pl. vin, figs. 6, 6a, 6b; 
1882). Without taking into account certain slight though recognizable internal dif- 
ferences, that species is distinguished by its narrower, more strictly parallel, and less 
frequently bifurcating branches, the obliquity of its zocecial apertures, and the lesser 
elevation and rigidity of the transverse interspaces. A nearer congener, perhaps, is 
the R. basalis (Stictopora basalis Ulrich, op. cit., p. 169, plate viii, figs. 4 and 4a), but 
the very frequent bifurcation of the zoarium characterizing that species serves to 
distinguish them at a glance.* For comparisons with R. trentonensis, R. fidelis, and 
other species described in this report see under descriptions of each. 
Formation and locality.—The typical form is extremely abundant in the middle and lower beds of the 
Trenton shales about Minneapolis and St. Paul, Minnesota. It occurs in these beds, but much less 
abundantly, also"at Cannon Falls, Lanesboro, Fountain, Preston and other localities in the southern part 
of the state, and at Decorah, Iowa. The var. mojor is fairly abundant at the three localities first named, but 
the Cannon Falls specimens are less robust than usual. From the Galena shales at Cannon Falls, I have 
identified with the species something over forty fragments. In these, however, the zowcial apertures are 
more oblique than usual. Respecting the Kentucky form, which I have heretofore referred to this species 
(14th Ann. Rep. Geol. Nat. Hist. Sur. Minn., p. 67, 1886), I prefer to await further investigations before 
expressing a conclusive opinion. This course seems the wisest also with respect to similiar forms from the 
Trenton rocks of Illinois, Tennessee, New York, Vermont and Canada. 
Mus. Reg. Nos. 5938, 5939, 5941, 5956, 5957, 7597, 7599, 7606, 7621, 7663. 
*A very good illustration of the necessity of thin sections for the determination of the generic relations of these bifoli- 
ate Bryozoa is furnished by my 1882 work on them in the publication cited. Had they been prepared of all the species therein 
defined. I would not have fallen into errors that now appear only too obvious. There I placed, for instance, Pachydictya 
acuta Hall, sp., Cystodictya gilberti Meek, sp., and Rhinidictya basalis under Stictopora, while Rhinidictya was founded, correctly 
enough, upon both external and internal peculiarities of R. nicholsoni. With sections I could scarcely have failed in detér- 
2 
mining the true position of these four species, 
