A MONOGRAPH OF THE EXISTING CRINOIDS 55 



In Pterocoma pennata from the Upper Jurassic we see a form exhibiting many 

 features in common with certain comatulids from the Upper Cretaecous, as well as 

 with the Thalassometrida. Gislen said that this early form does not show the least 

 approach toward the contemporary solanocrinids. Possibly this species represents a 

 type standing near the ancestral form of one or, perhaps, both the series mentioned 

 above — Thalassometrida and Notocrinida. The slender habitus, the long undiffer- 

 entiated rather scanty cirri, the numerous syzygies, and the prismatic pinnules with 

 strongly developed side plates and covering plates all indicate that Pterocoma pennata 

 should be placed very near the younger comatulid types and that it has nothing in 

 common with the solanocrinids. Although masses of this species, otherwise almost 

 perfectly preserved, are known, it is remarkable how very seldom the centrodorsal 

 has been noticed. This is presumably due to the fact that the centrodorsal very easily 

 becomes detached, and possibly also that at the animal's death it was left remaining 

 upon a still existent stem. In the only specimen among 81 examined in which he 

 found a centrodorsal preserved Walther observed and figured a large circular opening 

 in the center of the dorsal side. Gislen said he had observed a similar pore on the 

 dorsal side of a centrodorsal in a specimen he had examined. 



Gisl6n said that a circumstance speaking in favor of the Thalassometrida and 

 Notocrinida having reached the eleutherozoic mode of life at different times is the 

 very different point of time, at which their pentacrinoids become detached from the 

 stem — in the former very early, in the latter type at a very late stage. 



The Thalassometrida and the Notocrinida, according to Gislen, are the groups 

 that in the recent seas have retained most of the features belonging to the stalked 

 forms. These are especially numerous in the Thalassometrida — long cirri, some- 

 what rhombic in cross section; centrodorsal with the cirri in columns, sometimes in 

 an almost simple radial column; basals relatively well developed; the free dorsal 

 surface of the radials generally broad; the arm bases closely set and "wall-sided," the 

 arms ending abruptly with an abortive portion bearing rudimentary pinnules; the 

 pinnules prismatic to triangular in cross section; the side plates and covering plates 

 strongly developed; and the disk often closely studded with calcareous granules or 

 plates. 



The evolution of the palaeantedonids. — Gislen said that Semiometra is a final type 

 in the notocrinid series and constitutes a form transitional to the palaeantedonids, of 

 which the genus Palaeantedon is typical. Gislen did not believe it possible to doubt 

 that this genus represents a type corresponding to the recent antedonins, or in other 

 words to group a among the Macrophreata. This is evidenced by the centrodorsal 

 being closely set with numerous unsculptured cirrus sockets in alternating rows, by 

 the large centrodorsal cavity, by the lack of any radial pores or pits on the ventral 

 face of the centrodorsal, by the obliterated basals, by the large muscular fossae on 

 the articular faces of the radials, by the inward sloping of the radial articular faces, 

 by the slender oblique brachials, etc. The genus Hertha may be considered as a 

 predecessor of the palaeantedonid type, specialized and characterized by the develop- 

 ment of high narrow muscular fossae and a small radial cavity. The recent genus 

 Coccometra typologically corresponds to the genus Hertha. The genus Discometra 

 also comes very near the macrophreate type, although this type may prove to be a 

 heterogeneous one. Among the recent Macrophreata belonging to group a the Ante- 



