BRYOZOA. 125 
Rhinidictya.] 
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 incidentally only, 
and chiefly when comparisons are desirable. 
Rurnrwrerya murasiis Ulrich. 
PLATE VI, FIGS. 1-6, 12-18; PLATE VII, FIGS. 10-23, and 25-28; and PLATE VIII, FIGS. 1-3. 
Stictopora mutabilis (part.) ULRicH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minn., p. 66. 
Stictopora mutabilis, var. minor ULRicH. 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-celluliferous margins very scant, 
often practically wanting. Zocecia 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 zoccial 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 zoccial apertures are scarcely recognizable, the entire surface 
appearing as simply granulo-striate. In most cases the zocecial apertures 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- 
