A MONOGRAPH OF THE EXISTING CRINOIDS 45 



only the muscle of the aboral side is noticeably developed. In the articulation be- 

 tween the third and fourth pinnulars there is a great reduction of the sculpture of the 

 joint face, and from the articulation between the fourth and fifth pinnulars onward 

 we get typical synostoses. This gives the styliform appearance and the stiffness to 

 the pinnule. In certain of the Colobometridae there is a similar, though usually not 

 so extreme, stiffening of the proximal pinnules. 



In Austrometra the genital pinnules are provided with winglike processes, as is 

 the case in the Bathymetridae. Here, however, the processes are extended almost 

 horizontally so that the ossicle acquires a more disk-shaped appearance. 



Calometridae, Thalassometridae, and Charitometridae. In all these families the 

 pinnule segments after the third or fourth are decidedly triangular, though the more 

 proximal segments the second-fourth in the proximal pinnules are sometimes 

 rectangular. The triangular form of the pinnulars may sometimes be indistinct in 

 the proximal pinnules, these being more or less rounded, as in Neometra and Stylo- 

 metra. In these families the genital pinnules often have flattened segments with small 

 lateral processes that protect the gonads. 



In the family Calometridae the proximal pinnules have enormously enlarged first 

 and second pinnulars. The pinnule face on the brachial occupies a great part of its 

 ventral surface. The muscle attachment pushes its way rather deeply into the 

 brachial segment, as is usually the case also in the two other families. The transverse 

 crest of the pinnular face is twisted to an angle of at least 180 in relation to that of the 

 brachial face. The articular fossae between the proximal pinnulars are extraordinarily 

 well developed in all the proximal pinnules. The interarticular ligaments are narrow 

 or rudimentary. The union between the second and third pinnulars soon, however, 

 becomes much closer than those between the other segments, and after about P 7 

 acquires a synostotic character that is especially evident in Neometra. Processes are 

 also found on the earlier ossicles of the proximal pinnules, though less localized or 

 marked, in some of the Charitometridae, and outside of this group in Catoptometra 

 and in certain comasterids, as for instance Comanthina belli. 



Notocrinidae and Macrophreata. Gislen remarked that in regard to these forms 

 there is not much to be said. Within this group the muscle attachments are very 

 well developed; otherwise the pinnule articulations are fully typical, except for the 

 cases mentioned below. The distal segments of the distal pinnules are more or less 

 rounded, and their joint fossae are provided with the usual small dorsal fossa. 



The more important exceptions to the typical structure are the following. The 

 Heliometrinae have long proximal pinnules with very short segments. Here the 

 pinnular muscle fossae are very strongly developed and the hypertrophy culminates 

 between the distal pinnulars where, as in the Comasteridae, the most powerful muscle 

 is found on that side of the ossicle where the rudimentary comb is situated the 

 aboral side. Another exception to the normal structure is found in Isometra. In this 

 genus the third and fourth segments of the pinnules from about P 6 are greatly expanded, 

 evidently to serve as a protection for the gonad. The hypertrophy is most strongly 

 pronounced on the aboral side. 



Gislen had no opportunity for studying the pinnules of the Thysanometrinae or 

 Atelecrinidae. 



