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 Cystodidya. 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 

 Rhinididyn with affinities to R. mutabilis too close to admit of even specific distinc- 

 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 maculae, that the minute interstitial tubuli are exceedingly 

 numerous, there being often three longitudinal rows between adjoining zooecia. 

 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 (pi. VI) prepared from old exam- 

 ples of the typical form. The absence of horizontal lines in the lower part of the 

 walls maybe the result of imperfect preservation. 



This species, especially in its typical form, is to be regarded as closely allied to 

 R. nicholsoni Ulrich (Jour. Gin. Soc. Nat. Hist., vol. v. p. 170, pi. viii, 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 zooecial apertures, and the lesser 

 elevation and rigidity of the transverse interspaces. A nearer congener, perhaps, is 

 the R. basalis (Stidopom 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. Jidelis, 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 loss 

 abundantly, also at Cannon Falls, Lanesboro, Fountain, Preston and other localities in the southern part 

 of the state, and at Pecorah, Iowa. The var. major 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 y.ou'cial 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, Pachyiltetya 

 oeutallall, sp., CystodMya gUIterti Meek, sp., and Rhinidictya basalis under Stlctopora, while RhinMlctua was founded, correctly 

 enough, upon both external and internal peculiarities of R. nicholsoni. With sections I could scarcely have failed in deter- 

 mining the true position of these four species. 



