NIAGARA GROUP, 



195 



Genus ICHTHYOCRINUS (Conrad). 



This genus is characterized by Mr. Conkad (Jour. Acad. Nat. Sci. Philadelphia, Vol. viii, p. 

 279), as follows : 



" Column round, smooth ; canal small and round ; scapulae with the margins of the articu- 

 tions parallel, and somewhat imbricated." 



These characters are not sufficiently decisive, and alone do not enable one to distinguish the 

 fossil. 



The column is round, smooth and slender, having the articulating rays on the outer circum- 

 ference. The base of the cup shows five small triangular pelvic plates, succeeded by scapular 

 plates resting on the oblique upper edges of the pelvic plates : a second and third scapular 

 plate succeed in direct series, when subdivision takes place, which is repeated two or three 

 times at irregular intervals. The base of the cup, when the column is separated, presents a 

 tripetalous impression, which is probably of generic importance. 



This genus is known only in a single species ; but it possesses peculiarities of high interest, 

 showing its relations to crinoids with three pelvic plates ; while the arms proceeding in direct 

 line from the second series of plates, is characteristic of the lower forms with pentapetalous 

 calyx. 



588. 1. ICHTHYOCRINUS L^VIS. 



Pl. XLIII. Fig. 2 o -p. 

 Ichthyocrinus lavis. Conrad, Jour. Acad. Nat. Science, Vol. viii, pag. 279, pl. 15, fig 16. 

 Column slender, round and smooth, gradually enlarging towards the base of the cup, and 

 composed of alternating thicker and thinner joints ; base of the cup showing a tripetalous 

 impression (or three undeveloped pelvic plates surrounding the centre) ; pelvic plates five, 

 triangular, with wide bases ; on one side an interval between the pelvic plates, allowing one of 

 the second series to rest upon the column (see figs. 2 /", g) for a narrow space ^ second series 

 having the form and position of scapular plates ; lower side resting on the sloping sides of the 

 pelvic plates ; upper edge nearly parallel to the lower, giving the succeeding plates the same 

 form on the lower side, each series alternating with that laterally in juxtaposition ; first sub- 

 di\'isions taking place in the fourth or fifth plate from the pelvis ; second subdivisions occurring 

 on the sixth or ninth plate from the first one, and beyond this at irregular intervals ; plates 

 obtusely angulated on the exterior surface from the centre of the base to the sinus on the upper 

 edge, articulated on their upper and lower surface by marginal crenulations. On the side where 

 the plate of the second series rests on the column, there are two supernumerary plates inter- 

 calated on the lefthand side and above its upper lefthand angle. The subdivisions, though 

 apparently commencing by the division of a single plate, still show one edge elevated a little 

 higher than the other, so that the series are in their lateral relations alternate with one another. 



