52 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



casional plating of the disk, the covering plates in Nemaster and ComatUia, the oc- 

 casional appearance of the gonads (best 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 syzygial septa, the powerful hooks on the distal pin- 

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



The evolution oj the solanocrinid lype. The solanocrinids appear first in the 

 Middle Jurassic and become commoner in the Upper. Nevertheless they have 

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

 the cirrus sockets approaches more closely the pentacrinid type. Moreover, the 

 basals in the oldest forms are strongly developed. It may be supposed, therefore, 

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

 Palaeocomaster. 



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 

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

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

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

 mentioned may have 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 solanocrinids Archaeometra is the most primitive in regard to the 

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

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

 Cretaceous 15 columns of cirrus sockets become usual. In typical solanocrinids a 

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

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

 shallowness of the radial cavity and the sculpture of the cirrus sockets. 



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

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

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

 arms. In addition to these there are hi the Upper Jurassic, as well as in the Lower 

 Cretaceous, quite typical solanocrinids with monoserial arms. Among the younger 

 of these we find columns of large cirri on the flattened 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 

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

 careous tissue. 



It seemed to Gislen very probable that hi 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 the following specialized features the reduction of the 

 muscular fossae, distinctly indicated already in the Solanocrinidae, is often very 

 advanced and sometimes complete (Pontiometra and Stephanometra) ; the radial cavity 

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



