6 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



not strikingly higher than the number of odd segments, which, to judge from the 

 figures, occur pretty often. 



Gislen pointed out that in the cyathocrinids odd and even series occur mixed. 

 In the pentacrinites two separate series develop, one with an arm in the place of an 

 inner pinnule, and another with an arm in the place of an outer pinnule. In Telio- 

 crinus, with its division series composed of few elements, the new arm is usually, 

 though by no means always, in the place of an inner pinnule. In Diplocrinus, on the 

 other hand, the number of segments in the division series is odd (counting syzygial 

 pairs as units), and the new arm is therefore in the place of an outer pinnule. The 

 genus Cenocnnus to some extent forms a link between Neocrinus and Metacrinus, 

 since the outer series of divisions seem usually to have an even number of segments, 

 while the inner have an odd number. 



Among the comatulids, according to Gislen, it is only in the subfamily Capil- 

 lasterinae of the family Comasteridae that the form of pinnulation appearing in the 

 genera Metacrinus, Teliocrinus, and Neocrinus and represented by the formula 3(2+3) 

 reappears. So far as pinnulation is concerned this type is to be compared with 

 division series of 4(3+4) or 4(1+2, 3+4), though there the first syzygial pair has 

 coalesced into a single ossicle. All the other comatulids have either two or four 

 components hi their division series. In regard to pinnulation these are to be consid- 

 ered as one and two ossicles, respectively, since segments 1 and 3 in the division 

 series never have pinnules. Thus in the former case the new arm must be considered 

 as having developed on the outside of the main arm, in the latter case on the inside. 



The indifference as to whether it is an outer or an inner pinnule that is developed 

 into an arm that is found in certain pentacrinites, which is to be explained by the 

 shortening of the division series, recurs in certain primitive families of comatulids. 

 Thus in the family Charitometridae there are about as many forms with two as with 

 four components in the division series. The mixed type is also found in the Capil- 

 lasterinae, Comasterinae, and Zygometriclae (in the last two with a tendency for four 

 components to predominate), and in the Calometridae and Thalassometridae (though 

 most of the genera in the last two families have division series of two elements). The 

 family Himerometridae has almost exclusively forms with four components hi the 

 division series, while the Comactiniinae, Mariametridae (including the Stephanouie- 

 tridae), Colobometridae, and Antedonidae have taken the other path and have 

 practically always two elements in the division series. 



As the division series here have few elements, the young arms will be situated 

 near the base of the arms. As in the case of Neocrinus decorus an inner pinnule is 

 not more protected during development than an outer pinnule. Moreover, hi the 

 comatulids direct increase in the number of arms has been replaced by augmentative 

 regeneration, and the small arm regenerates, hidden between the bases of the arms, 

 are not very much exposed to breakage. In Gisjen's opinion this must be taken to 

 be the explanation of the indifference to the number of segments in the division series. 

 In the tendency of the axillaries to shift toward the proximal part of the arms, a 

 tendency that can be traced in the form series of all crinoids, there is this advantage if 

 the arms are exposed to a bending that will result in fracture, these fractures will 

 occur distally to the axillaries, and will therefore affect only single arms. On the 



