

the Morphology of the Blastoidea. 235 



and so causes much of the asymmetry of the calyx. One of 

 the three basals which occur in the calyx of every Blastoid 

 occupies a nearly dorsocentral position ; while the two others, 

 which are very closely united, are much elongated and form a 

 projecting fold where they meet the azygosone. They extend 

 upwards onto the lateral portion of the calyx and support the 

 azygos radial, just as the two long basals do in Eleathero- 

 crinus. This radial is thus thrust out of place, modified in 



< shape, and changed in appearance, and lacks the characteristic 



forked aspect of the radials of Blastoids generally. 



The plates on the ventral surface of Astrocrinus, formerly 

 described as the "convex crested plates"*, are the orals, 

 which are much larger in this type than in Eleutherocrinus ; 

 while the " spearhead plates " immediately surrounding the 

 mouth are not additional elements in the calyx, but merely 

 the constricted central ends of the orals. They resemble the 

 corresponding portions of these plates, which have been de- 

 scribed as u small rhomboidal pieces " in the summit of 

 Schizoblastus Sayi. Just as is the case in Eleutherocrinus, the 

 orals on either side of the azygos ambulacrum differ from the 

 other three in outline. 



* We much regret that the condition of our specimens is very 



unfavourable to the elucidation of the nature of the hydro- 

 spiral apparatus in Astrocrinus. That of Eleutherocrinus is 

 larger and well developed, as is shown by a specimen in Mr. 

 Wachsmuth's collection ; but, judging from the appearance 

 presented by some of the isolated radials of Astrocrinus , we 

 think it possible that the hydrospires may have been situated 

 partially or entirely within the substance of these plates, 

 somewhat as they are in Tricoelocrinus . 



Astrocrinus must have been a free and unattached form j 

 for it does not exhibit the slightest trace of any facet for the 

 attachment of a stem. The same is the case with Eleuthero- 

 crinus, as pointed out by Shumard and Yandell. This would 

 appear to indicate that the physiological condition presented 

 by Marsupites among the Neocrinoids is also reached by some 

 members of the older group of Blastoids. But Marsupites 

 has a regular dorsocentral plate, besides basals and under- 

 basals • while Astrocrinus and Eleutherocrinus seem to have 

 basals and nothing more. Certain species of Comatulce also 

 reach the same condition when mature, the cirri borne by the 

 young centrodorsal plate gradually falling off, while their 

 sockets become entirely obliterated. It is likewise probable that 

 some species of the Palaeozoic genus Agassizocrinus were free 



Quart. Journ. Geol. Soc. vol. xxxii. pp. 105, 106. 



