Catalogue of the Blastoidea. 



423 



Cystid, Cryptocrinus cerasus, and possibly in one Crinoid (Stephano- 

 crinus). Many Palaeocrinoids have a trimerous base, but according 

 to Wachsmuth and Springer ("Revision of the PalEeocrinoidea," 

 pt. iii. 1885, p. 10), the small plate is always in the right anterior 

 interradius (A— E). (See Woodcut, p. 422, Fig. 2, I.) 



The calyx of many Blastoids is marked by very well-defined 

 ridges, which start from the bottom of the basal cup and extend 

 upwards until they meet the radial lips at the distal ends of the 

 ambulacra. The general arrangement of these ridges is the same in 

 all Blastoids, and is perhaps best studied in StepJianocrinus in which 

 type they are very strongly marked (see Woodcut, Fig. 3, infra). 



Fig. 3. — Diagram to show the position of the ridges (1-8) on the three hasals 

 and the five radials (A, B, C, B, E) of iStephanocrinus angulatus (modified fromRoemer). 

 (The lettering of the plates is the same as in Fig. 2.) 



Lastly we may notice the very symmetrical grouping of the 

 hydrospires. " These organs occur in most Cystids, and perhaps even 

 in some Crinoids, but we know of no member of either group in 

 which their arrangement is at all like that which occurs in the Blastoids. 

 In this class they are restricted to the radial and interradial plates, 

 where they lie with their slits parallel to the ambulacra, and except 

 in the genus Codaster, there are always five interradial pairs of 

 hydrospire-groups. No Cystid whatever presents anything like this 

 regular distribution of the hydrospires, which often extend down on 

 to the basals, and even on to the under-basals, when such are present." 



" Considered as a whole, the Blastoids have the most regularly 



