32 CATALOGUE OF THE BLASTOIDEA. 



excavated in the direction of the rays, and the edge or crest between every two 

 sinuses is formed by the limbs of the radials. We do not hesitate to say that this 

 would be a most anomalous condition, and one altogether unlike that which occurs 

 in every other Blastoid. The oral crest of Codaster corresponds to the septum 

 which is so often found in the spiracles of Pentr mates with its extension outwards 

 against which the side plates rest, and in no single instance are these parts formed 

 by the limbs of the radials (PI. 1. figs. 5-7, 11). Both in Phcenoschisma and in 

 Codaster, which Wachsmuth admits to be the nearest ally of Heteroschisma, the oral 

 ridge is formed by the interradials (PI. XL figs. 1, 3, 5-7; PL XII. figs. 1-6, 8; 

 PI. XIII. figs. 1, 4, 8 ; PL XIV. figs. 9, 11, 12), as is also the case in Cryptoschi&ma 

 (PI. V. fig. 24 ; PI. XIII. fig. 20), Triccelocrimis (PL XIX. figs. 13, 15), Stephanocrinus 

 (PI. XIX. figs. 8-12), and Astrocrinus (PL XX. figs. 2, 11, 12, 14-16); and we have 

 good reason to believe that the same is the case in Heteroschisma also. According 

 to Wachsmuth l , the interradials of this type are " only partly exposed to view, the 

 visible part occupying a small space round the mouth, the concealed portions, which 

 give origin to the two inner hydrospires, being overlapped by the ends of two 

 contiguous limbs. There is in Heteroschisma, in the proper sense of the word, exte- 

 riorly ho ' oral ridge,' as the edge is here formed by the limbs. The limbs are 

 extended interradially into high pyramids with steep sides, of which two of the walls 

 form the sides of the radial sinus. The hydrospires are located, with the exception 

 of the two inner ones, within the pyramids, being exposed along the sides of the 

 sinus, nowhere connecting externally with the interradial plates." We do not 

 dispute that the walls of the radial sinus in Heteroschisma at its outer end are 

 formed by the limbs of the radials, just as in Cryptoschisma (PL V. fig. 24), Phceno- 

 schisma (PL XL figs. 1-3), and Codaster (PI. XII. figs. 1-G). But they take no part 

 in forming the oral ridge, which starts from the apex of each pair of radial limbs on 

 the exterior of the calyx, and is entirely located within the deltoid plates, just as in 

 the genera above mentioned (PL X. figs. 19, 20 ; PL XVI. fig. 1). If the somewhat 

 excavated summit be ground away, the concealed parts of the radial limbs on which 

 the deltoids rest come to be more or less completely exposed, just as they would in 

 Phcenoschisma nobile (PL XL figs. 1-3), or Stephanocrinus (PL XIX. figs. 8-12), 

 though this is less evident in the more flat-topped Codaster trilobatus (PL XII. 

 fig. 8) ; and it is, we think, in consequence of this that Wachsmuth has been led to 

 interpret his section of the summit of Heteroschisma as indicating that the oral crest 

 is formed by the radial limbs. 



In his type species, Heteroschisma gracile, " intermediate between the ambulacra 

 there are five pyramids, formed by the junction of the adjacent limbs of two 



contiguous radials, without the assistance of interradial plates." These 



hist are "small, and only partly exposed to view, one half or more of each 

 1 Proc. Davenport Acad. Nat. Sci. 18n3, vol. iv. p. 83. 



