1^4 THE PALEONTOLOGY OF MINNESOTA. 



[Rinidictya. 



Rhinidictya fidelis TJlrich. 



PLATE VI, PIGS. 7, 7a, 7b AND 8. 



Stietopor'a fldelis (part), Ulrich, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Sur. Minn., p. 68. 



To save repetition it will suffice to say of the external characters of this species 

 that they are exceedingly like those of the next described R. trentonensis, a slightly 

 greater width of the also more nearly quadrangular zooecial apertures, being rather 

 inconspicuous differences. 



In tangential sections the deepest parts show the prostrate portion of the zocecia 

 lying on, each side of the mesial laminae. The latter themselves may be shown as 

 in fig. 7b with the inclosed " median' tubuli." These horozontial tubuli seem to con- 

 nect with the vertical sets that form series separating the rows of zocecia. At first 

 the zocecia appear as simple quadrangular spaces, their width equalling about half 

 of the length. In the next stage these /spaces are divided by a line, transversely in 

 the central rows, and obliquely upward in the marginal ranges. This line represents 

 the incurving superior hemiseptum, which is developed to an unusual degree in this 

 species. In the stage immediately succeeding, the posterior half is covered, while 

 the open anterior part is gradually reduced in width till it assumes the elliptical 

 shape commonly presented by the "vestibular" portion of the zocecia. From now 

 on to the surface, the distance depending upon the age of the specimen, the section 

 exhibits little if anything to distinguish it from similar sections of other species. 

 There are rows of subelliptical apertures separated by thick interspaces, and between 

 the rows a dark, faintly flexuous line, which, when carefully examined, is found to 

 contain a series of minute pores. 



Vertical sections are highly characteristic, especially when they have been care- 

 fully prepared and show the primitive region of the zocecia in a satisfactory manner. 

 ThQ anterior side of the zocecial cavity is almost straight from the mesial lamina to 

 the superficial aperture. The posterior and upper side is concave and the curve 

 produced in front into a strongly developed hemiseptum, projecting over half the 

 distance toward the base of the anterior wall. An occasional complete diaphragm- 

 like structure may be detected crossing the tubular vestibule. All of these char- 

 acters are shown very well in fig. 8. 



As has been stated, it is not an easy matter to distinguish this species, by means 

 of external characters alone, from R. trentonensis, and until the observer has become 

 thoroughly familiar with the various forms of this genus occurring in the Minnesota 

 rocks, he is cautioned to secure the evidence of thin sections before he places much 

 confidence in his identification, of this species, at any rate. The strongly developed 



