82 DAVENPORT ACADEMY OF NATURAL SCIENCES. 



leaf-shai)ed disc, widi rounded extremities directed to the basal sutures. 

 The sutures are always slightly de[)ressed, and the parts covering these 

 depressions stand out so prominent that it appears almost as if, in this 

 case, exceptionally, the anchylosis had not been comi)leted; and in 

 fact I found two specimens in which the upper structure apparently 

 had partly fallen out, leaving in its place, at the surface of the basal 

 plates, a clover-leaf-like impression. The place to which the column 

 was attached is generally well marked; and in one specimen I found 

 remains of the proximal segment, which is exceedingly thin and deli- 

 cate, and, like the anchylosed joint, triangular in form, but the points of 

 the angles directed toward the sides of the upper one. 



I think this fully sufficient to prove that the so-called supplementary 

 basals in Pentremites consist of a columnar joint, anchylosed with the 

 basals and more or less completely absorbed into the plate, and that 

 the basals in the Blastoids generally were monocyclic and not dicyclic. 

 This seems to be also the opinion of Etheridge 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 ap- 

 proaches P/iiznoschisiiia, 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. Phcenoschisma, in 

 consequence, jjossesses ten groups of hydrosjMres, whilst Cadaster has 

 only eight. Further, the former genus has relatively 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 Phcenoschisina, 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 dif- 

 ferences in the so-called "oral plates" morphologically as important as 

 the difference in the number of the hydrospires. In the typical form 

 of Cadaster, including the more flat-topped species with large inter- 

 radials, the latter plates cover almost the whole of the truncate upper 



