the Morphology of the Blastoidea. 243 



other haivi, tlie number of American species may at any 

 moment be auo-mcnted by one or more of the doubtful ones. 

 Further, if certain forms described by Prof. James Hall are 

 subsequently found to be referable to this type, the genus will 

 have commenced its existence in the Upper Devonian of North 

 America. 



Genus SCHIZOBLASTUS, gen. nov. 



Gen. char. Calyx resembling that of Granatocrimis in form 

 and composition of the ambulacra. Oral plates of variable 

 size, but usually small. Spiracles double, being linear or 

 oval clefts between the lancet-plate and notched edges of the 

 orals, further removed from the peristome than in Pentremites^ 

 and not floored by the oral plates (as in the latter genus) ; 

 those in the anal interradius may or may not be confluent with 

 the anal aperture. 



Obs. That some restriction of the then existing genus 

 Granatocrimis was felt to be necessary by American palaeon- 

 tologists may be gathered from the following remarks by 

 Messrs. Meek and Worthen, who, speaking of Pentremites 

 melo and P. projectus^ say, " Both of these forms differ from 

 the typical species of Pentremites ^ in having each pair of 

 ovarian openings distinctly separate, instead of closely united, 

 with merely a thin septum between .... They constitute a 

 subgenus of Pentremites "*. The form of the calyx in Schico- 

 hk'.stus is, as a rule, pentagonal subglobose, or melon-shaped, 

 and when viewed in section is either simply pentagonal, as in 

 8. Sayi, or imperfectly decagonal, as in S. melo^ S. meJonoides^ 

 or S. missouriensis. The summit and base are much con- 

 tracted when compared wdth the apices of other genera. The 

 former is, generally speaking, to some extent flattened, whilst 

 the latter varies between very concave, truncate, or sliglitly 

 protuberant. In S. melo, S. Sayi, 8. melonoides, and 8. 

 missouriensis the concavity is but slight, in some individuals 

 so much so as to appear almost flat, whilst in *S'. ylaher it is 

 broad and apparently truncate. On the other liand, in 8. 

 granulosus the base is deeply concave, the radial plates being 

 quite hidden in the depression, so that they are invisible in a 

 side view. A similar concavity also exists in 8. pisum. 

 Lastly, in 8. neglectus and 8. project us the base projects to a 

 greater or less degree, and is visible when the calyx is placed 

 in an erect position before the observer. 



The radial plates resemble those of Granatocrinus by their 

 extreme variability in size. In 8. melo, 8. melonoides, and 



* Proc. Acad. Nat. Sciences Philadalpliia, 1861, p. 142. 



