126 THE PALEONTOLOGY OF MINNESOTA. 



IKhinidictyii. 



tudinally. about seventeen zocecia in 5 mm. Except in a variety to be considered 

 presently, the zooscial apertures always appear as direct. 



Vertical sections (plate VI, figs. 3 and 5) show that the zocecial tubes, in their 

 course from the basal (median) plate to the superficial aperture, form an angle of 

 about 50 degrees with the surface. In the primitive portion of the zooecia (i. e. that 

 part which lies on each side close to the median laminae) the posterior side curves out- 

 ward and forward so as to form a curve about equaling one-fourth of a circle. The 

 anterior extremity of the curve terminates abruptly at the primitive aperture ; from 

 this point to the surface of the zoarium, or in what has been described as the " vesti- 

 bular portion of the zooecium," the course of the wall is nearly straight. In a few 

 species of this genus (e. g. E. fidelis and R. minima) the junction between the "ves- 

 tibule" and the curved posterior side of the "primitive cell", is prolonged into a 



* 



short septum that I have designated as the " superior hemiseptum." In R. mutabilis, 

 however, this septum is but little, if at all, developed, the junction being merely an- 

 gular. W.hen the section shows the transverse interspaces (this is often the case 

 because of the great thickness of the interspaces in the vestibular region) they will 

 be seen to exhibit interrupted dark vertical lines. These represent the tubular 

 internal extensions of the minute granulations noticed at the surface. Stages in 

 the growth of the zoarium may also be determinable. These are marked by dark 

 lines, sharpest in the inner portions of the zoarium. 



In transverse sections the chief point of interest is the row of minute tubuli 

 that exists between the two parts of the duplex: mesial lamina. (See plate VI, fig. 6.) 



Tangential sections present a variety of appearances depending (1) upon the age 

 of the fragments sectioned, and (2) the depth beneath the surface represented in the 

 section. Using an. old example the section may be made, with judicious manipula- 

 tion, to show all the conditions through which the zoarium has passed, from the 

 beginning of the zooecia on the mesial laminae to their mouths. Taking such a sec- 

 tion, which, to be satisfactory, should not be less than 10 to 15 mm. long, the follow 

 ing features are likely to result : Starting with the mesial laminae, which will be 

 recognized as a faintly dark space, the first character worthy of notice are the " median 

 tubuli." These are represented by very delicate parallel lines, longitudinal in the 

 central third of the zoarium, but gradually diverging or curving toward its edges in 

 the lateral thirds. Though not yet clearly demonstrated in this species, I neverthe- 

 less assume it to be a fact (because of observations in other forms possessing such 

 tubuli) that the "mesial tubuli" connected with the minute tubes between the walls 

 of the zocecia, the surface extensions of which have been described as granules. (See 

 plate VI, fig. 18.) Just above the mesial laminae the section presents the basal or 



