152 Cincinnati Society oj JSTatural History. 



names have been given, are among the most remarkable and anomal- 

 ous of the series of Echinoderma that I have recently had the good 

 fortune to discover in the Trenton rocks of Kentucky. 



The almost perfect identity of the arrangement of the plates with 

 those of Hybocriiius; the three arms; the two ambulacra running down 

 the sides of the body as in the appressed arms of many Cystideans; 

 the valvular anal opening; the presence of a proboscis or ventral sac, 

 as indicated by the prominent az3^gous plate; and the uncovered area 

 of the vault adjacent to it, all taken together form a mixture of char- 

 acters not united in any fossil of this difficult class hitherto described. 



I have referred it to the Cystidese with some hesitation, but mainly 

 on account of the anomalous arrangement- of the ambulacral S3^stem, 

 three ra3^s of which are upright and two appressed, and on account of 

 the position of the anal opening, which is the same as that in Agela- 

 crinites, Hemicystites and Caryocrinus, with the slight modification in 

 position caused by the presence of a ventral sac. 



In this remarkable genus are thus combined, to an eminent degree, 

 the characters belonging to those palaeozoic forms so aptly designated 

 by Professor Dana as Comprehensive T^q^es. It is by the study of 

 such fossils that we may seek to eliminate the beginnings of those 

 earty lines of differentiation, that led to so man^^ distinct forms after- 

 ward, when their characters became individuall}^ separated, and were 

 borne by independent organisms; nor will the value of this study be 

 lost, should it hereafter be found that these characters have a sexual 

 reference. 



Locality and Position — Mercer count}^ Kentucky, in the upper part 

 of the Trenton Group. Seven specimens. 



Genus Hybocrinus, Billings. 



(Plate v., fig. 2, view of a nearly perfect specimen with arms; 2a, same specimen, an- 

 other view; 2b, larger specimen, lateral view; 2c, same specimen, azygous side, showing 

 crenulated and convex upper face of the a7>ygous plate. All natural size.) 



I have had the good fortune to discover a number of comparatively 

 well preserved specimens of this species, showing several of its char- 

 acters much better than Mr. Billings' t^qoes. The purpose which in- 

 duces me to figure them is, however, to enable the student to compare 

 Hybocrinus with Hyhocystites intelligentl3^ An examination of 

 figures 16 and 2c will .show the likeness on the azj'gous side. A like 

 study of figures Ic and 2a will assist in understanding the opposite 

 side : while figures la and 2& will show the difl'erences in the lateral 

 views. la showing the appressed ambulacral groove, cutting the 



