202 THE PALEONTOLOGY OF MINNESOTA. 
[Arthroclema. 
rows, arranged between prominent ridges that become stronger with age. A trans- 
verse arrangement also prevails, with about seven in 2.5 mm.; according to its length 
from seven to twelve cycles, the usual numbers nine or ten, are to be counted 
in each segment. Apertures ovate, oblique, the inferior border very prominent, 
spine-like. With a favorable light, two exceedingly delicate strive may be noticed 
in the longitudinal interspaces. These striv are not only generally present in 
species of this genus, but also occur frequently in species of Helopora. They are not 
shown in figs. 10 and 11, having been overlooked. 
As tertiary segments I propose to denominate a large number that seem to belong 
to this species but are more slender and differ in other respects from the ordinary 
forms of the secondary set. As a rule, they have tapering, subequal, rounded extrem- 
ities, are shorter, 2.5 to 3.3 mm. long, 0.8 mm. or less in diameter, five or six-sided, 
the latter generally, with the angles obscure in the youngest, but becoming fairly 
prominent with age. At the same time the raised border of the zoccial apertures, 
whieh at first is very thin, increases in strength and prominence. Six to eight 
cycles of zocecial apertures in the length of a segment. Figure 7, on plate III, 
represents an average segment of this set. In still younger specimens the angle 
ridges are less conspicuous, while in going the other way an almost continuous 
chain of variation from it to fig. 10 on plate II, can be selected from the material 
before me. 
This species is nearer A. pulchellum Billings, than I suspected at first. The only 
differences that I now would insist on are (1) the greater average length of the seg- 
ments, those of the second order especially, of the present species ; (2) the spine-like 
elevation of the inferior border of the zocecial apertures, and (3) the less frequent 
branching of the zoarium. In the Canadian species, of which I have some very good 
material, the primary segments have almost invariably two lateral articulating 
sockets, while the majority of those of the second order have one at least. In A. 
armatum, however, the primaries have only one or none, and the secondaries are in 
most cases without any. The two species of the lower shales, A. striatum and A. 
cornutum, are readily enough distinguished. Both have the lateral articulating 
sockets situated lower on the segments; the former has, furthermore, smaller 
zocecial apertures, more numerous and finer surface striations and only five instead 
of six ranges of zocecia in the secondaries. The segments of A. cornutum are of a 
different shape and much shorter. 
If complete zoaria of this species could be studied, it would not surprise me if 
they proved that the curved and tapering'jsegments which I have named //elopora 
mucronata, and which are found in the same beds, are really parts of this Arthroclema. 
