Prof. E. J. Chapman on the Genus Agelacrinites. 159 



this character is probably somewhat indefinite), the small border 

 plates in A. Billingsii form two or three circles, in A. parasiticus 

 they appear to occur only in a single row. 



2. Analytical Review of the Genus Agelacrinites and its included 

 Species. — The generic characters of Agelacrinites may be thus 

 defined : — Form circular ; stemless ; flat or concave below, and 

 somewhat convex above ; and covered by numerous small plates, 

 arranged in part irregularly, and in part in regular order. The 

 definitely arranged plates form five rays (ambulacral areas ?), 

 which originate at the centre of the upper side of the body. 

 These rays are either short and straight or long and curved. 

 They are also composed of a double series of small polygonal 

 plates, interlocking along the central line of ray ; or, otherwise, 

 of a single (?) series of plates (Roemer's A. Rhenanus). The 

 irregularly arranged plates are elliptical or circular, variable in 

 size, very numerous, thin, scale-like, and imbricating ; or imbri- 

 cating at and around the margin of the disciform body, and joining 

 by their edges in the more central part of the disk. The mar- 

 ginal plates are commonly very small, and, in some species, are 

 separated from the more central plates by a circle of compara- 

 tively large pieces. In the centre of one of these (interam- 

 bulacral ?) spaces, and about midway between the apex of the 

 body and the margin, is situated an orifice covered by a pyramid 

 of five or more (moveable ?) plates. The apex itself, or centre 

 and origin of the rays, is covered by a single circular plate, 

 or is surrounded by five or ten angular plates — these latter 

 constituting the first plates of the rays. Characters of the 

 under side of the body, position of mouth, &c, not definitely 

 known. 



From this definition, it is clear, as, indeed, is universally allowed, 

 that Agelacrinites belongs to the Echinodermata. In the pre- 

 sent state of our knowledge, however, it is impossible to refer it 

 satisfactorily to any one of the admitted orders or families of 

 that class. With the Crinoids proper, and the Blastoids, it 

 appears to have only general affinities j but with the Cystideans 

 it is evidently closely connected : more especially by the posses- 

 sion in common of a pyramidal orifice or so-called anal pyramid. 

 It differs from the Cystidean structure, nevertheless, in many 

 important respects : the peculiar rays, the imbricating plates, 

 the absence of a stem, for example, are essential points of dif- 

 ference. The imbrication of the plates serves to connect it, 

 through the genus Protaster, with the Euryales or the Ophiurians ; 

 and the conformation of the rays, in certain species, appears to 

 afford another link in support of this view. But is it not equally 

 related to the Echinida ? After a careful consideration of the 

 subject, I cannot refrain from hazarding an opinion that the 



