1869,] BILLINGS — STRUCTURE OP CRINOIDS, ETC, 289 



under, and another partly under the arm, five others being fully 

 exposed. S, S. Lyon has described a species under the name of 

 C. alternatus in the " Geology of Kentucky,'' vol. iii., p. 494, 

 from the Devonian rocks of that State, which closely resembles 

 C. Canadensis, but is still distinct therefrom. Speaking of the 

 structure of the summit, he says : " the depressed triangular 

 intervening spaces are filled with seven, or more, thin pieces, lying 

 parallel to the pseudambulacral fields, articulating with the 

 summit of the second radial, and the prominent ridge lying 

 between the pseudambulacrse. These pieces were evidently 

 capable of being compressed or depressed; the"^oi««" at the 

 lateral junction of the second radials is in some specimens folded 

 over toward the mouth, so as to entirely obscure these triangular 

 spaces by covering them." This important observation proves 

 that, even in the same species, the hydrospires may be either 

 partly or wholly concealed under the arm. The " point ^' to 

 which Mr. Lyon alludes is seen above, in fig. 11, just below the 

 letter 6. It is the same as the " small triangular pyramid^^ in 

 C Canadensis. It is evident that (supposing the shell to be 

 flexible) if these points were to be drawn inward, the movement 

 would gradually cause what remains exposed of the hydrospire to 

 be covered, until at length it would be entirely concealed under 

 the arm. The five points would then be situated in the angles 

 between the five ambulacra, as they are in the genus Penfremites, 

 fig. 15. The concealment of the hydrospires may also be the 

 result of the widening of the arm. This is well known in P. 

 caryophyllitm DeKoninck (P. Orbignyanus according to Roemer), 

 P. Schultzii De Ver., and several other species. In these the 

 apices of the pyramids remain near the margin, but the hydrospires 

 are nearly covered by the wide arms. This is shown in fig. 11, 

 where the ends of the fissures of the hydrospires are seen along 

 the sides of the angular ridges which extend from the apices of 

 the pyramids to the angles between the arms. I do not think 

 that such species can be referred to Pentremites, and if I had 

 specimens before me, instead of figures only, I would most prob- 

 ably institute a new genus for their reception. 



Our specimens of G. Canadensis are well preserved, and show 

 the characters of the arms perfectly. After many careful ex- 

 aminations under the microscope, I can state positively that in 

 this species the so-called " pseudambulacral fields" have no pores. 



The markings that have hitherto been mistaken for ambulacral 



