l62 NEW YORK STATE MUSEUM 



Two reasons have induced me to describe this embryonic form 

 as a new species and give it the self-explanatory name of Embryo- 

 crinus problematicus. The first of these is because of the arm 

 structure. 



The gum process has enabled me so many times to reproduce 

 sutures where others had long failed to detect them (as, for instance, 

 in the anal and radianal plates of Palaeocrinns stratus; see New 

 York State Museum Bulletin 149, plate 6, figure 1, and in recent 

 work on Steganoblastus) that I must place some confidence in its 

 ability to show such structures where they really exist. These 

 crinoid arms show no trace of them. In studying the stereogram 

 here presented comparison should be made with the four plates 

 already mentioned as accompanying Doctor Clarke's recent annual 

 report. It is doubtless true that some of the white partings in 

 our present illustration are due to cleavage planes in the calcite, but 

 it must be noted that the plates of the theca show but few such 

 planes and their white partings reveal true structural characters. 

 It must also be noted that in the pedicellariae of Urasterella 

 medusa the spines and spinlets actually broke up into disks on 

 decay. If the interpretation of structure here given be correct, 

 then primitive arms in crinoids developed, so far as their main 

 ossicles were concerned, much as did spines, and from such a 

 linear series of mixed disk-shaped and wedge-shaped ossifying 

 centers either a uni serial or a biserial arm might be evolved. The 

 factors determining the ultimate outcome would in large measure 

 be due to the amount and kind of motion given this arm during its 

 period of evolution. 



The second reason for presenting this specimen to the public 

 is because it forms so fine an example of light penetration made 

 possible by mounting in a transparent medium and under a cover- 

 glass, thus doing away with the surface scattering of light from 

 the myriad points of an unpolished surface. 



Embryocrinus problematicus, new genus and species 

 Theca subconcial, 0.75 mm wide at base, across basals 1.66 mm, 

 height of cup 1.8 mm. Apparently four or five infrabasals, five 

 basals and five still larger radials, two of which are clearly seen and 

 one at the left carrying an arm is also apparently present. From 

 radial to first fork of arms 2.5 to 2.8 mm. Column 0.6 mm wide, 

 apparently of five series of plates, nine in 0.6 mm. These micro- 

 scopic plates of the column are fairly well shown in the photo- 



