EF. Billings on the structure of the Crinoidea, etc. 79 
is, in general plan, a section across one of the ambulacra of a 
Pentremite. On examining nearly all the published figures of 
species of this genus I find that there is a series of forms 
which exhibit a gradual passage, from those ‘with the hydro- 
spires almost entirely exposed, as in fig. 8, through others in 
which they are crowded more and more “under the arms, until 
at length they become iu internal, 
8. 9, 
772 7 
2g et 
en 
e posterio of ead’ pe 9. Section across the fyikebeie’ from d, t 
7 a, isthe place of the arm. 10. The eg contracted as a “4 
11. Summit of Pentremites caryophyllatus D 
In C. acutus, fig. 8, only a small Sek of the hydrospire 
is cosoented gees the arm. In 0. anadensis, a new species 
lately discovered in the shales of the Hamilton group in Can- 
ada West, each of the four interradial spaces, in which the 
: hydrospires are placed, is excavated, in such a manner as to 
- form a small sar pg p ramid, with two of its faces slo 
ing asa towar es of the two adjacent arms. On 
these two nope ri goers) the brdres ospires, which appear to 
have one fissure entirely under, and another partly under the 
_ arm, five others being fully exposed, 8. 8. Lyon has described * 
_ aspecies under oS name of CU. alternatus in the “ Geology of 
_ Kentucky,” vol. . 494, from the Devonian rocks of that 
| 
: 
Fig. 8. beta of C. acutus McCoy, m, v, mouth and vent; d, d, suture ce 
., a 
udambulacre. 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 1 hydrospires may be 
