A MONOGRAPH OF THE EXISTING CRINOIDS 25 



types, on the other hand, a stout and solid structure is of less consequence, if only the 

 levers are powerfully built. It is therefore difficult to avoid the supposition that the 

 thickness of the arm bases bears a certain relation to the creeping habits of the comas- 

 terids. As the ligament fibers are much more ultimately connected with the calcareous 

 mass of the ossicle than are the muscle bundles, as the muscles in these types are rather 

 poorly developed, and as it must be presumed that the ligamentary elements possess 

 a capacity for active motion, Gisle"n sees no objection to accepting the hypothesis 

 that an arm of the type mentioned must answer better the demand for an efficient pull 

 and push lever than is the case with the more graceful and delicate arms found in the 

 other comatulids that are better adapted for swimming. 



Gisl6n noted that a large number of the oldest (Jurassic) comatulids also had very 

 stout arm bases that were often thickened proximally, where the same tendency to 

 biseriality seen in Comatidella and Comatula appeared, although often it went much 

 farther. Here, too, the muscular attachments, as in all the older Articulata, are weakly 

 developed in comparison with the ligamentous connections. Gisle"n said that one can 

 not help thinking that these phylogenetically old comatulids, which had fairly recently 

 given up their sedentary habits, had not yet gained any great capacity for swimming, 

 but were chiefly creeping types. On the other hand the antedonids, which are good 

 swimmers, would then be a later type, more suited for a free and active mode of 

 existence. 



SOLUTION OF THE REVERSION PROBLEM 



The causes of the shortening of the pinnule-bearing side of the proximal brachials 

 in certain comatulids according to Gisle'n are the following: 



The degree of the effect caused by an articulation's capacity for flexing the arm 

 as a whole both ventrally and laterally increases as the position of the articulation 

 becomes more proximal. The greatest capacity for flexion must therefore be con- 

 sidered to exist in the most proximal brachial articulations. In the proximal articula- 

 tions the interarticular ligament on the distal articular face of the brachials is developed 

 much more strongly on the pinnular side than on the antipinnular side. As a result 

 of the encroachment of the pinnule socket, which is fairly large in the proximal portion 

 of the arms, the interarticular ligament of the pinnular side, and also the transverse 

 ridge, is displaced dorsally on the pinnular side of the brachial's distal articular face. 

 On the proximal articular face of the succeeding brachials it is the end of the trans- 

 verse ridge on the antipinnular side of the brachial that is displaced dorsally. In 

 other words, we find in the dorsal part of the articulation a certain likeness to a 

 synarthry, although a very oblique one. The dorsal end of the transverse ridge, which 

 runs between the great interarticular ligament and the dorsal ligament, is now produced 

 so that a process from the proximal margin of the more distal brachial fits into a notch 

 in the distal margin on the more proximal brachial. We thus get something very 

 similar to an oblique synarthrial projection where, as in the latter, the synarthrial 

 process is directed backward. At the same tune as this prolongation of the proximal 

 margin of the distal brachial takes place posteriorly, the point of attachment for the 

 great interarticular ligament spreads out on the lateral inner side of the increased 

 attachment surface. As the projection from the more distal ossicle fits into the more 

 proximal ossicle on its pinnular side, it encroaches upon the breadth of the pinnular 

 side of this brachial, which thus becomes narrower than the antipinnular side. On the 



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