132 THE PALEONTOLOGY OF MINNESOTA. 



[Rhinldiulya,. 



this is not the case. The latter are also exceptionally well preserved and show that 

 the apertures are really rather strongly oblique, with a slight "lip" at the posterior 

 border. Not in these specimens, even, have I detected satisfactory evidence of the 

 presence of the row of granules on the longitudinal ridges usual in species of this 

 genus. Yet, as is shown by thin sections, the minute inter-zooecial tubuli, whose 

 superficial extension forms the granules, are developed in the usual manner. 



The obliquity of the zooecial apertures allies this species to the larger R. nichol- 

 soni, but not closely enough to cause confusion between them. The zooecia are 

 larger'in that species, there being thirteen to fifteen where we have seventeen to 

 eighteen in this form. It also resembles R. patipera and R. minima, but they are dis- 

 tinguished : the first by having more ranges of zooecia with the apertures in several 

 of the marginal rows on each side of the branches oblique ; the second by its smaller 

 zoo3cial apertures and much wider granulo-striate interspaces. 



Formation and locality. Comparatively rare in the lower third of the Trenton shales at Minneap- 

 olis, St. Paul and near Fountain, Minnesota. 





 EHINIDICTYA MINIMA Ulricli. 



PLATE V, FIGS. 13-18. 



Rhinidietya minima ULRICH, 1890. Jour. Cin. Soc. Nat. Hist., vol.xii, p. 183, fig. 8. 



Zoarium small, branches 0.8 to 1.2 mm. wide, commonly 1.0 mm., bifurcating at 

 intervals of 2 or 3 mm. Zooecia in five or six longitudinal rows, increasing to seven, 

 eight, or nine before bifurcation takes place ; sixteen in 5 mm. lengthwise. Size and 

 shape of apertures, and character of interspaces, varying with age. The enlarged 

 figures on plate V represent the usual appearance of the oldest examples. In these the 

 zooecial apertures are small and narrow-elliptical (about 0.11 mm. by 0.06 mm.) and the 

 interspaces very wide, with the granulose ridges projecting but little above the level of 

 the peristomes surrounding the apertures. Under a glass of low power the interspaces 

 appear as rather flattened, and marked with straight or slightly flexuous longitudinal 

 striae. Under a higher power the striae resolve into rows of small papillae, with one 

 continuous series, a little stronger than the others, separating the apertures into 

 longitudinal rows, and one or two short series in the slightly depressed spaces 

 between the ends of the apertures. When in a good state of preservation, a row of 

 granules, rather smaller than the others, is found to crown the peristomes as well. 

 These were overlooked in drawing fig. 15. In younger examples the principal longi- 

 tudinal ridges are relatively higher, causing the zooecial apertures, which in these 

 cases are wider, and the intermediate spaces to appear as set in shallow channels. 

 Not infrequently the peristomes of succeeding zooecial apertures are connected in a 



