MONOGRAPH OF THE EXISTING CKINOIDS. 



515 



young in various stages of development. In their " Seaside Studies," published 

 in 1865, his son, Mr. Alexander Agassiz, and Mrs. Elizabeth Gary Agassiz men- 

 tioned this species for the first time and gave a rough sketch of the pentacrinoids. 

 Though the specimens still exist in the Museum of Comparative Zoology at 

 Cambridge, they have never been described. 



On January 30, 1885, the Albatross dredged in 21 fathoms off the coast of 

 Yucatan a large number of the young of this species, to the cirri of which many 

 pentacrinoids were attached. Several years ago I examined these specimens super- 

 ficially, making notes of the more striking features, and laid them aside for 

 future study. 



When Mr. Frank Springer was completing his monograph on the Crinoidea 

 Flexibilia I suggested to him that he take the material and study it in connection 

 with his work on the group. This he did, and with the assistance of Dr. Herrick 

 E. Wilson he prepared a description of these pentacrinoids which is so compre- 

 hensive and so detailed that all that is necessary here is to give a resume of 

 their results. 



None of us, and none of those to whom we have shown the specimens, have 

 been able to detect any trace of inf rabasals in this form. 



There were in all 200 specimens of the larvse, which were found clinging to 

 the cirri of a number of young individuals. The crowns of the larger ones, the 

 stems of which pass upward between the arms of the foster parent, appear on the 

 ventral side of the latter as a circlet of closed or opened buds lying close down 

 upon the proximal pinnules. 



These larvse show almost every stage from that of a nearly closed cup of 

 basals and orals, with faint incipient radials and a long tapering column, through 

 the development of anal, radial, interradial, and perisomic structures, arms, pin- 

 nules, and cirri, to the stages characterized by the diminution and ultimate resorp- 

 tion of the orals and interradials, and the loss of the column. 



The very small pentacrinoids measuring from 0.31 mm. to 1.85 mm. in the 

 diameter of the calyx were attached to the cirri of the foster parent by the usual 

 form of primitive terminal stem plate. 



The youngest larvse show a calyx composed almost exclusively of basals and 

 orals, the latter closed and standing quite erect, and a column with the segments 

 increasing in length distally. 



Very small patches of calcareous reticulation consisting of a few spicules, 

 which are the beginnings of the radials, have appeared in the rhombic spaces 

 between the corners of the basals and orals ; but in one of these spaces the radial 

 is wanting, and in its place there is a somewhat larger plate similar in composition 

 to the others, which is attached to the anal end of the intestine, now plainly visible 

 on the outer side of the calyx. This plate is the radianal, which now lies in the 

 position corresponding to that which the radials occupy in the other areas, at the 

 base of the area to be occupied by the right posterior ray. 



The calcareous reticulation is so thin and translucent that by transmitted light 

 the complete alimentary system is plainly visible. It consists of the esophagus, 



