52 BULLETIN B2, UNITED STATES N'ATK i\ai. MUSEUM 



mal plating <>f the disk, the covering plates in Nemaster and ComatUia, the oc- 



nal appearance of the gonads dies! developed in the posterior radii) in the arms, 

 and the creeping mode of locomotion. Specialized features are the reduction of the 

 cirri, the numerous arms and Byzygial septa, the powerful hooks on the distill pin- 

 nules, and the comb on the proximal pinnules and sometimes also on the distal ones. 



flu evolution oj tfu solanocrinid iype. — The solanocrinids appear first in the 

 Middle Jurassic and become commoner in thi Upper. Nevertheless Dhe3 have 

 centrodorsals thai are more primitive than in Palaeocomaster, and the appearance of 

 il irrus sockets approaches more closely the pentacrinid type. Moreover, the 



- in i! Idesl forms ore strongly developed. It may be supposed, therefore, 



according to Gislen, that the Bolanocrinids developed into comatulids later than 

 PaJUu oeomasU r. 



It is also possible that the solanocrinids are derived from a different branch of the 

 pentacrinids. In Palaeocomaster, and in its descendants the recent comasterids, there 

 i- found a strong tendency toward reduction of the cirri, as in Seirocrinus. There is 

 never any reduction of the cirri in the Solanocrmidae, which always have the cirrus 

 Bockets large and powerful, as in Pentacrinus. Possibly, therefore, the type first 

 mentioned maj bai e descended from a form more nearly related to the former penta- 

 crinid genus, and the second type from a form more nearly related to the later one. 



Among the Bolanocrinids Archaeometra is the most primitive in regard to the 

 basals, which are very powerful. The cirrus sockets are still rather few and large. 

 In Solanocrinu8 the number of cirrus rows and columns is increased, and in the Lower 

 Cretaceous l."> columns of cirrus sockets become usual. In typical solanocrinids a 

 centrodorsal with cirri in alternating rows is never attained. On the other hand, the 

 genus Solanocrinus seems to show more primitive features in respect to the size and 

 -hallow ne<s of the radial cavity and the sculpture of the cirrus sockets. 



Gislen remarked it may now he asked whether the solanocrinids have completely 

 died out, or whether they survive among the recent forms. The former has probably 

 been the case with a number of clumsy forms from the Upper Jurassic with biserial 

 arm-;. In addition to these there are in the Upper Jurassic, as well as in the Lower 



Cretac i-. quite typical solanocrinids with monoserial arms. Among the younger 



of these we find columns of large cirri on the Battened centrodorsal; but the sculpture 

 of the cirrus sockets has almost disappeared, and the muscular fossae are low and 

 broad and are possibly sometimes absent. The centrodorsal cavity still continues 

 small and shallow; the radial articular faces lean only inconspicuously inward, and the 



I cavity is therefore large — possibly it is beginning to be filled by spongy cal- 



It seemed to Gislen \i-ry probable that in the recent suborder Mariametrida we 

 have the recent descendants of the solanocrinids. The difference is that the sculpture 

 of the cirrus sockets in the younger forms has been still further reduced and therefore 

 at its highest consists only of an areola, and that the cirri, 'generally at least, alternate 



distinctly. There are further tin- following specialized features the reduction of the 



musculai distinctly indicated already in the Solanocrinidae, is often very 



advanced ami sometimes complete (PonHometra and Stephanometra); the radial cavity 



aerally filled up with spongy calcareous tissue; the syzygial septa have increased 



