BRYOZOA. 199 



Arthroclema 



placed near the upper end. Zooecia in five or six longitudinal rows, with small 

 oblique apertures, the posterior margin prominent ; arranged in transverse rows, 

 with six the usual number in the length of the segment ; short segments (2.5 mm.) 

 have five. Surface distinctly striated lengthwise, often thrown into strong, but 

 never sharp, ridges between which the zocecial apertures are arranged. The latter 

 are always difficult to see. In drawing fig. 30 I overlooked them entirely, having 

 mistaken certain depressions, which are often met with in these segments, for them. 

 Since freeing the specimen from the matrix I find that it has really five rows of 

 apertures, both longitudinally and transversely, of the same size as shown in fig. 33. 

 With this correction the figure may still be of aid in the identification of the species, 

 because it shows the striation of the surface better than the original of fig. 28. It 

 should be added, however, that the latter shows the usual shape of the segments 

 better. 



Secondary segments five-sided, 2.7 to 3.2 mm. long, the length depending upon 

 the number (six or seven) of transverse rows of zooecial apertures, 0.35 to 0.5 mm. in 

 diameter, the ends usually a little wider, the upper flat, the lower rounded. Zocecial 

 apertures in five longitudinal and six or seven transverse rows, small, oblique, drawn 

 out above, their ends widely separated. Interspaces finely striated lengthwise, the 

 angles sharp or rounded, formed by a raised central line and one or two similar lines 

 on each side of it. An articulating scar or shallow socket has been observed just 

 above one of the first cycle of zooecial apertures, but their presence in the segments 

 of this set is to be counted as an unusual occurrence. For this reason it is more 

 than probable that most of the segments which are next described as tertiary are 

 really young or terminal joints of the second order. 



Tertiary segments about 3.0 mm. long, less than 0.35 mm. in diameter, very 

 slender, four or five-sided, with sharp angles, the lower end rounded and tapering 

 slightly, the upper sharply truncate. Zooecia in four or five longitudinal rows, in 

 six, but oftener in seven cycles, their apertures small, oblique, the raised margin 

 highest posteriorly and running out on each side to the angle ridges. Behind each 

 aperture usually a pair of very delicate striae, the conditions being very much as in 

 Helopora spiniformis (see plate III, fig. 4). Angles formed by a single raised line. 

 These segments are distinguished from those of the second order by their greater 

 tenuity, more oblique zocecial apertures, and absence of striae on the sides of the 

 angle ridges. 



As stated in the paragraph preceding the last, it is possible that many of the 

 segments just described as of the tertiary set really represent the young stage of the 

 secondary set. In that case it is probable thai the four-sided joints only are ter- 

 tiaries. 



