240 Messrs. R. Etheridge, Jan., and P. II. Carpenter on 



peristome, like those already referred to in Eleacrinus. The 

 paired linear plates in the ambulacra we believe to be single, 

 and to represent the lancet plates of other Blastoids. They 

 seem to be usually much eroded and to have a strongly marked 

 median groove, which has been taken for a suture. Even 

 when these plates are preserved the side plates of the ambu- 

 lacra are generally missing ; but since Hall has discovered 

 specimens of S. angidatus still retaining ambulacral appen- 

 dages like those of other Blastoids, we see no reason to doubt 

 the existence of side plates and outer side plates. In fact, the 

 former have been described in 8. pidchellus by Miller and 

 Dyer. This species, together with S. gemmiformis and 8. 

 osgoodensisj is much more like other Blastoids than the better- 

 known 8. angulatus, which has a very peculiar external form, 

 owing to the great development of the interradial processes. 





The most striking fact, however, in the morphology o 

 Stephanocrinus is the apparent absence of hydrospires. Each 

 radial sinus is closed below by downward extensions of the 

 plates at its sides ; and even when the lancet plates are re- 

 moved there is absolutely no trace of any hydrospires, such as 

 are so visible on the more or less sloping sides of the radial 

 sinuses in Phcenoschisma and Codaster. But we know that 

 Stephanocrinus had ambulacral appendages like those of other 

 Blastoids ; and we believe these to have had a ciliated ventral 

 surface along which currents of water flowed down into the 

 hydrospires through the marginal pores of the ambulacra. 

 The appendages of Stephanocrinus were doubtless of the same 

 nature as those of other Blastoids ; and it is to be expected 

 that hydrospires were also present, though they may have 

 been, and probably were, actually within the substance of the 

 radials as in the distal portions of the ambulacra of P. co- 

 noideus and throughout the greater part of their length in 

 Triccelocrinus and perhaps in Astrocrinus. 



The absence of any external indication of hydrospires is a 

 very marked feature of Stephanocrinus ; and it is therefore 

 with no little surprise that we have found Prof. Hall writing 

 even as late as 1879, and again last year* : — 8. gemmi- 

 formis. " The structure of the calyx and the arrangement 

 of parts on the summit and ambulacra appear to be iden- 

 tical with Codaster In the structure of the body 



at least there are no differences which appear to be of generic 

 importance between Stephanocrinus and Codaster." As a 

 matter of fact, however, these two genera are as widely diffe- 

 rent as any two of the symmetrical Blastoids can well be. In 

 all members of the group the- structure of the " body " is 



* Indiana Report, pp. 270, 280. 



I 







