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 striae may be noticed 

 in the longitudinal interspaces. These striae 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 1 1, 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 difier 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, 0.3 mm. or less in diameter, five or six-sided, 

 the latter generally, with th^ angles obscure in the youngest, but becoming fairly 

 prominent with age. At the same time the raised border of the zooecial apeitures, 

 which at first is very thin, increases in strength and prominence. Six to eight 

 cycles of zooecial 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 zooecial 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 

 zooecial 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 tapermg]segments which I have named Helopora 

 mucronata, and which are found in the same beds, are really parts of this Arthroclema. 



