644 



THE CRINOIDEA CAMEEATA OF NORTH AMERICA. 



B.C. 



Dorsal cup. 



F.D. 



Ventral disk. 



b. 



basals. 



R. 



radials. 



I. 



costals. 



ir. 



interradials (in part interbracliial, in 





part interambulacral). 



ia. 



interambnlacrals of the regular sides 



XX. 



interambulacrals of anal side. 



0. 



orals. 



cp. 



covering plates. 



Remarks. — The PlatycrinidfB are the typical family of a sub-group which 

 is distinguished from all other families of the Camerata by having the brach- 

 ials and interbrachials only slightly represented in the dorsal cup. The 

 lower brachials, although incorporated into the calyx, belong in part to the 

 cup and in part to the disk, and have more or less the form of free arm 

 plates. From this we conclude that the Platycrinidse represent phylogenet- 

 ically a less developed stage of the Camerate Crinoid, in which the lower 

 arm plates i-emained in an immature condition, their small size giving rise 

 to the interposition of three plates to fill ujj the iuterradial space in the first 

 row, instead of the usual single one. The lower interradials, therefore, are 

 for the most j)art neither strictly interbrachial nor interambulacral, but may 

 be considered as belonging to both. This is not invariably the case, however. 

 In a remarkable undescribed species of Platymnus from the Mountain lime- 

 stone of England, the costal and the two distichals are connected with those 

 of adjoining i-ays by five or six heavy plates, which are strictly interbrachial 



