American Paleozoic Bryozoa. 149 
The genus as above defined includes, beside the typ2 species, five 
other forms, four of which are described in this number. On account 
of their simplicity of structure, inexperienced collectors will probably 
find some difficulty in distinguishing one from the other. It must, 
however be borne in mind, that the more simple these organisms are, 
the more important are their variations. In separating them from 
each other, the characters principally to be taken into consideration 
arethe following: The growth of the zoarium; the size of the cells, and 
thickness of their walls ; the presence or apparent absence of small 
(interstitial?) cells, and their distribution if present ; the size and 
number of the spiniform tubuli ; and lastly, the disposition and num- 
ber of the diaphragms crossing the tubes. 
For comparison I have figured (Pl. VI., fig. 5) a portion of a tan- 
gential section of D. aspera. It shows the isolated and remarkably 
developed spiniform tubuli characteristic of this species. The zoarium 
of D. aspera consists of a large basal expansion attached to foreign 
objects, from the surface of which from one to five sub-cylindrical 
primary branches arise, that vary in diameter from .4 to .8 of an inch. 
These are frequently and irregularly divided, until the zoarium has 
attained a height of four or five inches. The terminal branches are 
sometimes much flattened, usually however retaining a sub-cylindrical 
form, while their diameter varies from .2 to .d inch, The surface is 
usually elevated into low and rounded monticules, four or five of 
which may be counted in the space of .5 inch. They are occupied by 
cells a very little larger than the average, scattered among which it is 
common to find a limited number of much smaller cells, that may be 
of the nature of interstitial tubes. Unless badly worn the spiniform 
tubuli constitute a conspicuous feature of the surface. About five 
occupy the space of .1 of an inch. Longitudinal sections show that 
the diaphragms are wanting in the axial region, and remote in the 
peripheral. The spiniform tubuli are readily recognized by their 
thick walls and slender cavity. As is the casein all the species of the 
genus, the walls often assume a peculiar beaded appearance, due to an 
alternation of light and dark shades of sclerenchyma. 
Like the other species of the genus D, aspera, is restricted in its 
vertical range, which is from 300 to 325 feet above low water mark in 
the Ohio river, at Cincinnati, O. At this height fragments are quite 
common. 
Until lately I was under the impression that Nicholson’s D. attrita 
might be advantageously regarded as a distinct variety of D. aspera, 
