252 PALAEONTOLOGY OF ILLINOIS. 



plate, and that the basals in the Blastoids generally were mono- 

 cyclic and not bicyclic. This seems to be also the opinion of Eth- 

 eridge and Carpenter, although they state distinctly that they wish 

 to leave the question for further consideration. 



HETEROSCHISMA, Wachsmuth, Nov. gen. 



The form under consideration is closely allied to Cadaster, and 

 approaches Phanoschisma, Ether, and Carp. The latter, according to 

 Etheridge and Carpenter, differs from Cadaster in the following 

 points: "In the partial exposure of the hydrospire slits, and in 

 their presence in the anal interradius, as well as in the four others. 

 Phfenoschisnia in consequence possesses ten groups of hydrospires, 

 whilst Cadaster has only eight. Further the former genus has rela- 

 tively smaller orals than the latter, and it may possess outer side- 

 plates to the ambulacra." (Ann. and Mag. Nat. Hist., April, 1882, 

 p. 227.) 



Heteroschisma stands intermediate between the two forms, it agrees 

 in the above characters with PJuenoschisma except that it has but 

 eight groups of hydrospires in place of ten. 



Admitting that the difference in the number of hydrospiral groups 

 alone is sufficient for a separation from Cadaster, the intermediate 

 form must be placed either together with that genus, or be arranged 

 under a new name. I follow the latter course, as I consider the 

 structural differences in the oral plates as important as the numer- 

 ical difference in the hydrospires, the more as they involve other 

 important structural complications. In the typical form of Cadaster, 

 including the more flat-topped species with small orals, the latter 

 plates cover almost the whole of the truncate upper face of the 

 body, resting with their lateral sides against the inflected upper 

 part of the limbs. The oral ridges consist of more or less strongly 

 marked edges or elevated ridges, which occupy nearly the full width 

 of the body. To both sides of the ridges are placed the hydro- 

 spires, all located within the limits of the oral plates, only small 

 portions of them being continued along the sides of the limbs. In 

 the more clavate form with small orals, for which I propose the 

 name Heteroschisma, the orals are partly only exposed to view; the 

 visible part occupies a small space around the mouth ; the con- 

 cealed portions, which underneath give origin to the two inner 

 hydrospires, are overlapped by the ends of two contiguous limbs. 



