NIAGARA GROUP. 



185 



CRINOIDEA OF THE NIAGARA GROUP. 



In the clescriptioQ of the fossils of this family from the Niagara group, it has been necessary 

 to establish eight new genera among the true Crinoidea : these have been founded upon 

 important parts of the structure, or the relations of one part to another, showing the ne- 

 cessity of such a course. In nearly every one a full detail of the structure is given, so that a 

 comparison with others may be made in all its parts not depending on a simple figure of the 

 fossil. The details of structure in the Genus Ichthvocrixus of Conrad show some interesting 

 relations to the crinoids with three pelvic plates. 



Two species related to Caryocrincs have been added to that type, which, when more fully 

 known, may perhaps show a transition to the Cystidea of this group. 



The number of species of Crinoidea, Cystidea, and one asteroid form now known in this 

 group, are twenty-seven, which have been studied from important parts of their structure ; 

 while several others are known from fragments of columns, and other parts of the animal. 



Genus HOMOCRINUS (nor. g^cn.). 



[Gr. o(xo?, similis, and xpivov, Ulium.] 



Crinoidea having the calyces composed of three series of simple plates, each series consisting 

 of five plates ; sometimes one or more irregular plates intercalated between the scapular or 

 third series of plates on one side ; arms proceeding from the summit of the third series of plates, 

 simple or bifurcating, composed of a single series of plates, without tentacula. 



The crinoids constituting this genus have been referred to Poteriocrinus or Cyathochinus, 

 the structure of which genera are somewhat different. An examination of specimens belonging 

 to these genera shows the necessity of separating those silurian species hanng the characters 

 given above. The two species given as PoTERiocRiias in Vol. I. Pafeontology of New-York, 

 should be placed under this genus, having the same simple structure and arrangement of plates. 



579. 1. HOMOCRINUS PARVUS (n. sp.). 



PL. XLI. Fig. 1 a -/. 



Minute, slender, structure microscopic ; composed of five elongated pehic plates, which 

 are surmounted by an equal number of elongated costal plates ; scapular plates short, minute, 

 and surmounted by minutely slender elongated arms ; arms composed of very elongated joints, 

 which are thickened at their extremities ; column composed of joints, of which the thickness 

 nearly equals the width of the column. 



This is an extremely minute species, of which two individuals occur on a small fragment of 

 stone. It is probably in a mature state, since the form and proportion of the plates is very dif- 

 ferent from any other species observed in the group. 

 [Paleontology — Vol. ii.] 24 



