262 Mr. E. Billings on the Structure of 



which they are crowded more and more under the arms, until 

 at length thej become altogether internal. 



In C. acutus (fig. 8) only a small portion of the hydrospire 

 is concealed under tlie arm. In C. canadensis^ a new species 

 lately discovered in the shales of the Hamilton group in Ca- 

 nada West, each of the four interradial spaces in which the 

 hydrospires are placed is excavated in such a manner as to 

 fonn a small trian(/ular j^p'anud, with two of its faces sloping 

 down toward the sides of the two adjacent arras. On these 

 two slopes are placed the hydrospires, which appear to have 

 one fissure entirely 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 dis- 

 tinct 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 tlie 

 pseudambulacra. These pieces were evidently capable of 

 being compressed or depressed: the ^ point'' at the lateral 

 junction of the second radials is in some specimens folded 

 over toward the mouth, so as to entirely obscure these trian- 

 gular 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 li\ it is the same as the " small triangular jryramid'''' 

 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 Pentremites (fig. 15). The concealment of the 

 hydrospires may also be the result of the widening of the 

 arm. This is well shown in P. caryoph/llatus, De Koninck 

 (P. Orhignyamis^ according to Roemer), P. Schultzii, DeVern., 

 and several other species. In these the apices of the pyra- 

 mids 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 spe- 



