MORPHOLOGY 43 



Mesozoic and Recent crinoids with a very few exceptions. It may be com- 

 parable to a stage in the senile condition sometimes found in the Recent 

 crinoids, where the transverse ridge becomes obsolete. These articulations are 

 well shown by various figures of Forbesiocrinus nobilis on Plate XXIV. Occa- 

 sionally in the Flexibilia a vestige of the transverse ridge is seen, as in Lecan- 

 ocrinus (Pis. I, fig. 5; II, fig. 3) ; but in these cases there are no paired muscle 

 or ligament fossae. It will be noted that while in Forbesiocrinus nobilis the 

 articulation on the faces of the brachials is as above described, that upon the 

 distal face of the radials is different in not being paired (PI. XXIV, fig. yb). 

 This raises the question whether the so-called radial is homologous with the 

 radial in genera with only 2 IBr, or whether it may not rather be the represen- 

 tative of the lower half of a compound radial, analogous to the radianal. Un- 

 fortunately it is impossible to obtain the evidence with which to follow up this 

 question in other genera. It is extremely difficult to obtain disintegrated speci- 

 mens in which these joint surfaces are clearly shown; the only case in which 

 I have found them being that of Forbesiocrinus. 



It is now generally accepted by authors that in all crinoids the arms mor- 

 phologically begin with the first plate above the radials, 1 the latter term being 

 thus restricted to the lowermost circlet of radially situated plates ; these plates 

 (Radials) appear at an early stage in the larval crinoid as small points at the 

 angles of the basals and orals (PL B, figs. 1-3), and they undergo considerable 

 development before any sign of brachial structures is visible. The plates which 

 finally succeed them in the rays are treated as primary brachials up to and 

 including the first axillary. In some Inadunata, including most of the Larvi- 

 formia, and occasionally among the Recent crinoids, there is no bifurcation, 

 and the arms are mere continuations of the primary brachials, with no line of 

 demarcation between them. But in the great majority of crinoids, both fossil 

 and recent, the ray divides at some point, usually on the second or third plate. 

 This is the case in all Camerata and all Flexibilia, among which forms with 

 five simple arms are unknown. 



The brachials of the Flexibilia, both primary and those of higher orders, 

 are to a large extent characterized by an arcuate curvature of their transverse 

 sutures, which imparts a peculiar aspect to the crown not commonly found in 

 any other fossil group. The proximal margins bend downward in a more or 

 less deep median curve, overlapping the distal margins of the plates next be- 

 low, which are often hollowed to meet it (PI. XXVI, figs, lb, c). This curva- 

 ture is usually most pronounced in the distal portion of the arms, appearing in 

 a smaller degree lower down; and sometimes it is entirely wanting. It is least 



1 Mr. Clark is now of the opinion that the radials are not fundamentally true calyx plates, but are 

 structurally more closely related to the ossicles of the brachial series immediately following them; the 

 reasons for this view, based upon the study of the comatulids, will shortly be given by him in full. 

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