PLATYCRINID^. 645 



in position ; and a similar structure, in a less degree, is to be observed in a 

 few American species. 



Tlie above general character of this family is shared with it by the Hexa- 

 crinid^, 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 Platycrinidte 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 Hexacrinidce, on the other hand, 

 have a large anal plate interposed between the two posterior I'adials, 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 Platycrinidfe embrace six genera, of which about one hundred and 

 eight species have been identified : — seventy-three in America and thirty- 

 five in Europe. Although introduced in the tipper 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 

 Platycrinida3 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. Coecocrinus and Culicocrinus also represent stages of the growing 

 Platycrinus; both have unusually large orals, which in the former are 

 symmetric, in the latter asjmimetric, and there is but a single row of in- 

 terradial plates between orals and radials. The case of llmsupiocrimcs is 

 somewhat different ; it has small asymmetrical orals occupying the centre 

 of the disk, numerous ambulacral and interambulacral plates, and highly 

 developed biserial ai'ms. The genus has all the characteristics of a mature 

 Platycrinoid except that it has the round stem of the young Plcdf/crimis, but 

 with a large quinquelobate canal ; these are in fact the only characters upon 



