r,v,- A- . n BEYOZOA. 125 



Khimdictya.] 



York and Canada, the specimens at each of these localities are marked by individual 

 peculiarities, causing their identification to be, in some cases at least, unsatisfactory 

 and generally rather difficult. Nothing less than monographical work can do the 

 genus justice. Manifestly, even if possible in the present state of our knowledge, 

 such work would be out of place here. 



I shall therefore largely restrict my remarks to the Minnesota forms, while 

 those occurring in other sections of the country will be mentioned incidenta,lly only, 

 and chiefly when comparisons are desirable. 



Rhinidiotya mutabilis Ulrich. 



PLATE VI, FIGS. 1-6, 12-13; PLATE VIL FIGS. 10-23, and 25-28; and PLATE VIII, FIGS. 1-3. 



Stictopora mutabilis fpart.) Ulkich, 1886. Fourteenth Ann. Eep. Geol. Nat. Hist. Surv. Minn., p. 66. 

 Stictopora mutabilis, var. minor Ulkich. Ibidem, p. 67. 



Zoarium a branching bifoliate stipe, varying considerably in width and superfi- 

 cial aspect. 



Typical form : — In the commonest or typical form, the branches vary in width 

 from 2.3 mm. to 3.2 mm., and in thickness from 0.7 mm. to 1.9 mm.; they divide 

 dichotomously at intervals varying from 7 to 16 mm., but on an average a bifurca- 

 tion takes place every 10 to 12 mm.; edges generally sharp, but with age become 

 blunter as the stipes increase in thickness ; non-cell uliferous margins very scant, 

 often practically wanting. Zooecia arranged in from ten to eighteen rows ; the usual 

 number is fourteen or fifteen, but just beneath a bifurcation it generally exceeds 

 twenty. Between the rows are straight longitudinal ridges, angular and crowned 

 with a single series of small granules in well preserved young and average examples, 

 thicker, rounded, and with stronger and more numerous granules in old examples 

 (see plate VII, fig. 10). In young examples again the spaces between the ends of the 

 apertures are slightly depressed, causing them (the apertures) to appear as openings 

 in the bottom of shallow channels. In such specimens (see plate VII, fig. 15) the 

 interspaces are comparatively thin and the zooecial apertures correspondingly large, 

 the long diameter of the latter being about 0.20 mm., and the short or transverse 

 diameter about 0.12 mm. With age the transverse diameter may be reduced to less 

 than 0.5 mm., while the channelled appearance becomes obsolete in the general thick- 

 ening of the interspaces. In a few fragments, apparently representing the condition 

 of extreme age, the zooecial apertures are scarcely recognizable, the entire surface 

 appearing as simply granulo-striate. In most cases the zocecialapertures in one or 

 more of the marginal rows are directed upward and outward. Measuring trans- 

 versely, about eleven of the central rows in 2 mm, (extremes ten and twelve); longi- 



