A MONOGKAPH OF THE EXISTING CRINOIDS 23 



In the comatulid brachials special cases sometimes occur in which the greater 

 length of the pinnular side is pronounced from the beginning; more often, however, 

 a series of discoidal segments follows the fourth brachial, or the reversion phenomenon 

 occurs. 



INFLUENCE OF THE PINNULATION ON THE OBLIQUENESS OF THE BRACHIALS 



In the distal portions of the arms the position of the pinnules evidently has a 

 controlling influence upon both the outer and the inner obliqueness of the brachials. 

 The pinnular side of the brachial becomes more strongly developed; this causes its 

 outer obliqueness. The pinnule's point of insertion enlarges the pinnular side of the 

 articular face; this causes its inner obliqueness. 



As examples of the decisive role that the position of the pinnules plays in regard 

 to obliqueness Gislen mentioned a specimen of Pectinometra flavopurpurea in which 

 pinnules appear on the right side of both the ninth and tenth brachials, the proximal 

 and distal borders of the tenth brachial being, as a result, parallel; and a specimen of 

 Heliometra glacialis in which on one arm the ninety-second brachial bears two pinnules 

 of normal size (19-20 mm. long) and has the distal articular face perfectly symmetrical 

 and lying in a plane perpendicular to the longitudinal axis of the arm. 



Gislen remarked that in certain cases it seems as if the idea that phylogenetic 

 causes may have played a certain part in bringing about obliqueness cannot altogether 

 be dismissed. Thus in Atelecrinus, which lacks pinnules as far out as the twelfth or 

 fifteenth brachials, if the arm division and pinnulation were the only causal factors, 

 it might be expected that straight articulations would be found between at least the 

 fourth and twelfth brachials. This seems to be the case regarding interior, though 

 not exterior, obliqueness. The side of the brachial that would have borne the pinnule, 

 if a pinnule were present, is in fact always longer than the opposite side. It is pre- 

 sumable that phylogeny must be invoked here in order to explain the exterior oblique- 

 ness of the brachials. The lateral flexibility, except at the synarthrial articulation, 

 here seems to be extremely slight, so that there is no trace of reversion in this case. 



On the proximal brachials the pinnule socket always more or less invades the 

 articular face, either intruding between the muscular and interarticular ligament 

 fossae, or lying ventral to the muscular fossae. On more distal brachials the pinnule 

 socket may wander out on to the lateral portion of the brachial so that is it completely 

 isolated from the articular face of the brachial, or it may lie on the ventral side of the 

 brachial and therefore, as in the proximal portion of the arm, ventral to the muscular 

 fossa. In the former case the brachials are more or less elongated with markedly 

 oblique articulations, while in the latter the brachials are usually short and discoidal. 

 Here, however, the size of the pinnule base plays a certain role. On account of the 

 insertion of the pinnule bases the Calometridae ought to be placed in the latter 

 group. However, the extraordinarily stout proximal pinnule segments do not permit 

 such a pronounced shortening of the brachials as is seen in the Mariametridae and 

 Himerometridae . In the Calometridae the pinnule attachment is extended at the 

 expense of the muscular fossa of the pinnular side, which is reduced and acquires a 

 more horizontal position on the distal side of the articular face. 



In other cases where the pinnule has a ventral origin the shortening of the brachials 

 (ontogenetically as well as phylogenetically) may lead to discoidal brachials on which 

 the proximal and distal articular faces are parallel. If, on the other hand,' the origin 



