210 PROOEEDINQS OP THE ACADEMY Of [1886. 



in connection with the ambulacra, is beneath the covering plates 

 through the canals or tunnels which we have described. These 

 passages, which are closed at the bottom by the deflected lateral 

 edges of two adjoining interradials, as in the case of Cyathocrimis, 

 grow deeper and narrower on approaching the oral plates ; while 

 toward the arms they widen and divide, transmitting a branch to 

 each main division of the rays. All this goes to prove that the 

 passages are tubular canals, such as we find in the Paleeocrinoi- 

 dea, and that the apparent resemblance between the ambulacra of 

 Stephanocrinus and those of the Blastoids, to a large extent, is 

 superficial. 



The oral 133-ramid, which is rather symmetrical in outline, 

 occupies the very centre of the ventral surface. It is composed 

 of five pieces of nearly equal size, which meet in the centre, and 

 are so closely connected by suture that the lines of union are 

 rarely seen in the specimens. A less close union seems to have 

 existed toward the interradials, for the oral plates are but seldom 

 preserved, when we find in their place a nearly circular opening. 

 The radials extend to the full height of the coronal processes, 

 and resemble in their form closely the forked plates of certain 

 Blastoids ; while in Hall's figure (N. York Rep., vol. ii, PI. 48, 

 figs. 1 h and 2/) they appear like the radials of a Platycrinus. 

 Hall evidently mistook the cracks which so frequently are found 

 at the base of the interradial processes, for sutures, and supposed 

 the united limbs of two contiguous radials, formed a deltoid- 

 shaped interradial plate. 



The most interesting fQa.t\xve of Stephanocrinus unquestionably 

 is the quinque-partite oral pyramid, which, we think, gives us 

 valuable information regarding the undivided oral plate in other 

 groups of the Palaeocrinoidea. It was suggested by us on p. 55, 

 that probably the central or oral plate of the Palteocrinoidea 

 primitively consisted of five pieces, of which the suture lines 

 gradually were obliterated by deposition of new material. In 

 support of this theory we could only refer to the parallel cases of 

 the basals and underbasals among which similar modifications took 

 place not only palaeontologically by anchylosis of one or more of 

 the plates, but also in the growing Crinoid by deposition of lime- 

 stone at the outer surface of the plates, as in the case of Edrio- 

 crinus and Agassizocrinus. We were unable at that time to point 



