1885.] NATURAL SCIENCES OF PHILADELPHIA. 259 



are so universally represented throughout this group, should be 

 totally absent in this genus, seems to us not very probable. 

 Yet the central opening which should contain them is so small, 

 compared -with the space taken up by them in Platycriniis-am\ 

 allied forms, that it seems almost impossible to have been occu- 

 pied by seven or more plates. Besides, there is not a single 

 instance known to us, in which either the summit plates or the 

 covering pieces were obliterated in the specimen, leaving at the 

 same time the interradials in position, as we find it in all these 

 specimens. This leads us to the conclusion that in Coccocrinus, 

 as in Platycrinus, the five interradial series had been separated 

 laterally to their full length, but that the disk covered by the 

 summit plates had not been raised to the surface as in that genus, 

 leaving an open gap and lateral clefts permanently as in Holopus, 

 with the exception, however, that in the latter genus the clefts 

 are formed between the orals. According to our interpretation, 

 Coccocrinus represents phylogeneticalby a transition form between 

 Culicocrinus, in which the interradials are still closed and its 

 summit plates and covering pieces subtegminal, and Platycrinus 

 in which they are incorporated with the calyx. This is the only 

 explanation which meets all difficulties, and brings these genera, 

 with regard to the distribution of the plates, under the same rule 

 with the other Palseocrinoids. 



The genus Symbathocrinus is morphologically a much higher 

 form than either Coccocrinus or Haplocrinus, not only because 

 it had better developed arms, but also well developed summit 

 plates. Its summit had never been observed until we removed 

 the arms in ve^ perfect specimens, and succeeded in laying 

 bare the whole ventral surface. It consists of eight plates, 

 four large proximals, which, together with three other plates, 

 along the azygous side, form a closed ring around a very con- 

 spicuous central piece, and these again are enclosed by ten or 

 more smaller pieces, which rest upon the highly elevated articular 

 facets of the radials. 



These outer plates (there may be one or two additional ones 

 toward the azygous side) are smaller than the proximals ; five of 

 them are placed radially, the others interradially. In the first 

 specimen which we dissected, and which was sent to Dr. Carpenter 

 for stud}-, the lateral sutures between the smaller plates could not 

 be distinguished. That plates were interposed between the radials 



