34 CATALOGUE OF THE BLASTOIDEA. 



make out any difference in the relative positions of the hydrospires in Codaster 

 Hindei, or of C. pyramidatns on the one hand (PI. XII. figs. 1-6) and those of 

 C. alternatus, var. elongatus (PL X. figs. 19, 20) and C. gracilis (PL XVI. fig. 1) on 

 the other, which is not fully accounted for by the varying depth of the radial sinus. 

 Neither can we see that the relative position of the hydrospires in C. Hindei, 

 C. pyramidatus, or C. trilobatus (PL XIII. figs. 1-4, 8) is any further away from 

 the mouth than in C. alternatus, var. elongatus (PL X. figs. 19, 20). 



The deltoid plates of Stephanocrinus are worthy of special notice, more particularly 

 as no figures of them in the type species (S. angulatus) have yet been published, and 

 they were not known to occur in the genus at all until Hall 1 published his later 

 figures of S. gemmiformis. This type has comparatively low coronal processes, which 

 are principally formed by the limbs of the radials, though minute deltoids appear 

 externally at their upper ends and form the descending oral crests on their inner 

 faces. As regards S. angulatus, Hall's original description has been generally aban- 

 doned in favour of that which was subsequently given by Eoemer 2 . The latter 

 author nowhere described deltoid plates as actually present (partly, perhaps, because 

 lie believed Stephanocrinus to be a Cystid and not a Blastoid), but he saw indications 

 of sutures extending downwards from the anal opening towards the ambulacra at its 

 sides ; and we now know that these are the radio-deltoid sutures which are visible in 

 some of our specimens in other parts of the summit as well, though they are not 

 always easy to see, except in a particular light. Each of the four coronal processes 

 is thus divided into an outer portion, formed by the contiguous limbs of two adjacent 

 radials, and an inner portion, the interradial, which reaches up to the apex of the 

 coronal process as in S. gemmiformis. The anal opening is more or less distinctly 

 visible in one of the processes, where it occupies the angle of the radio-deltoid 

 suture ; and in the corresponding position in some of the other processes there are 

 occasional indications of a minute perforation (PL XIX. figs. 8-10), which is more 

 distinct in specimens that have been slightly ground. Sometimes also traces of 

 another and larger perforation of the same kind are visible, as shown in PL XIX. 

 figs. 8, 10 ; and in summits that have been rubbed down the two openings in each 

 coronal process appear as one which has a somewhat keyhole-like form, the outer 

 end being the larger. It is probable that these openings are in some way related to 

 the hydrospires ; but upon this point we have no information at all. Neither are 

 we quite clear as to the position of the interdeltoid sutures beneath the ambulacra ; 

 for even when the ambulacral structures are .removed, as shown in three rays of the 

 specimen represented in PL XIX. fig. 8, we cannot make out any sutural lines 

 between the proximal ends of the deltoid plates such as are so visible in Pentremites 



1 "The Fauna of the Niagara Group in Central Indiana.'' 28th Ann. Rep. New York State Mus. Nat. 

 Hist. Albany, L879, pi. xiv. fig. 2. 



3 Archiv f. Naturgesch. 1S50, Jahrg. xvi. lid. i. p. 870. 



