PLATYCRINIDZ&. 645 
in position; and a similar structure, in a less degree, is to be observed in a 
few American species. 
The above general character of this family is shared with it by the Hexa- 
crinidew, which were placed in the same family by all writers on Crinoids up 
to 1885, when we separated them. The two groups, though more closely 
related to each other than to any other family, are nevertheless readily dis- 
tinguished by the form of the base, depending upon the presence or absence 
of an anal plate within the ring of radials. ‘The Platycrinide have no such 
anal plate, the radials being in contact at all sides. The base is therefore 
a pentagon, and is composed invariably of three unequal plates, the sutures 
between them being always directed to the right posterior, anterior, and left 
antero-lateral radials. The interbasal sutures are, however, very often anchy- 
losed and invisible from the exterior. The Hexacrinidx, on the other hand, 
have a large anal plate interposed between the two posterior radials, resting 
by its full width upon the edge of the basal cup; they consequently have a 
hexagonal base, which in that family consists either of two or three equal 
plates. | 
The Platycrinids embrace six genera, of which about one hundred and 
eight species have been identified :— seventy-three in America and thirty- 
five in Hurope. Although introduced in the upper Silurian, the family was 
not prolific in forms until the age of the Subcarboniferous, when, especially 
in the Lower Burlington limestone, and in their typical genus, Platycrinus, 
they reached their climax ; — abounding in number and variety, and in 
beauty almost surpassing the Crinoids of any other group. 
The modifications which took place in the course of time among the 
Platycrinidse are very slight; and in fact the young Cordylocrinus from the 
Niagara, with its uniserial, sometimes zigzag arms, which it retains during 
life, resembles most remarkably the immature Platycrinus of the Subcarboni- 
ferous. Ooccocrinus and Culicocrinus also represent stages of the growing 
Platyermus ; both have unusually large orals, which in the former are 
symmetric, in the latter asymmetric, and there is but a single row of in- 
terradial plates between orals and radials. The case of Muarsupiocrinus is 
somewhat different; it has small asymmetrical orals occupying the centre 
of the disk, numerous ambulacral and interambulacral plates, and highly 
developed biserial arms. The genus has all the characteristics of a mature 
Platycrinoid except that it has the round stem of the young Platycrinus, but 
with a large quinquelobate canal ; these are in fact the only characters upon 
