170 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



material, having been preserved in alcohol, does not show such minute histological 

 details with sufficient distinctness. The pore really appears to be closed, but this may 

 perhaps be due to contraction; in any case this point deserves closer investigation. 



The stone canal appears to have the form indicated by Russo, but it could not bo 

 made out with certainty. The inner half of the parietal canal has tliickcr walls than 

 the other part. On the inner side of the parietal canal is a small body which apparently 

 represents the primary gonad. The axial organ was found to be only slightly developed. 



In the earlier stage the elements of the skeleton arc just beginning to form. As 

 in other species of Antedon and in Tropiometra the orals and basals do not lie exactly 

 opposite one another. At this stage there are 8 to 10 columnals. A single very small 

 plate, just a small grain lying within the circlet of basals, represents the first rudiment of 

 the infrabasals. 



In the next stage the skeletal elements have enlarged considerably and have the 

 usual fenestrated structure. In this stage within the basals there are to be seen 4 plates 

 much smaller than they and with only one or a few holes. These are the infrabasals. 

 Usually there are 4 of them, but sometimes only 3, or more rarely 2; in no case were 

 5 found. They are about equal in size and there is no indication that any one of them 

 might be a double plate so that the real number would be 5. There are about 18 

 columnals at this stage. 



Pentacrinoid. — Dr. Mortensen found the pentacrinoids of this species attached to 

 corallines growing on the rocks in the same localities where the adidts lived. 



In the youngest pentacrinoids he secm-cd, the basals enclose the proximal ends of 

 the orals; the latter are distinctly concave along the middle line, the edges being prom- 

 inent. The infrabasals can no longer be seen distinctly, but on dissociating the 

 calyx very carefully they arc seen Ij-ing within the basals about the proximal colum- 

 nals where thej' are found on the succeeding pentacrinoid stages. There are still only 

 IS columnals of which the distalmost three to four are very short, nearly spherical, and 

 the middle ones more elongate; in all of them the primitive annulus is very prominent. 



In the next stage the radials have appeared; the radianal, which appears shortly 

 before the radials, lies almost in the mid-radial line, the corresponding radial lying almost 

 in the middle of the region between the orals and basals. The orals are no longer em- 

 braced by the basals, and their proximal ends arc beginning to turn outward. A couple 

 of small newly formed columnals are seen at the proximal end of the stem, which is now 

 composed of 20 columnals. 



In the next stage the orals have become separated from the basals so that a strip of 

 naked perisome is seen between the adjoining ends of the plates. The radials have 

 enlarged and a prominent knob has appeared on then- distal border to which the IBr, 

 are attached; beyond the IBr, the IBrj have appeared. The small columnals at the 

 pro.ximal end of the stem have broadened somewhat, but have not increased in length; 

 apparently no new columnals have been formed. 



The pentacrinoid of this species, according to Dr. Mortensen, is especially char- 

 acterized by its tentacles, which are long and slender and provided with calcareous 

 spicules— thin, bow-shaped, finely spinous structures which are usually arranged in a 

 single series in each tentacle. In specimens mounted in Canada balsam these serially 

 arranged spicules are very conspicuous and curious objects. They are found equally 

 well developed in the tentacles of the adults. 



In a slightly more advanced stage the IBro is distinctly bilobed, and the IBr, is 



