NIAGARA GROUP. 
185 
CRINOIDEA OF THE NIAGARA GROUP. 
In the description 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 Ichthvocrinus of Conrad show some interesting 
relations to the crinoids with three pelvic plates. 
Two species related to Caryocrinus 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 (nov. gen.). 
[Gr. o[j,og, similis, and xpivov, lilium .] 
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 Cyathocrinus, 
the structure of which genera are somewhat different. An examination of specimens belonging 
to these genera shows the necessity of separating those silurian species having the characters 
given above. The two species given as Poteriocrinus in Yol. I. Palaeontology of New-York, 
should be placed under this genus, having the same simple structure and arrangement of plates, 
arms, etc. 
579. 1. HOMOCRINUS PARVUS (n. sp.). 
Pl. XU. Fig. 1 a 
Minute, slender, structure microscopic; composed of five elongated pelvic 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 
