288 THE CANADIAN NATURALIST. [Sept. 



who first showed that they were also identical in structure there- 

 with. On comparing one of those with that of the cystidean 

 Pleurocystites, fig. 5, we at once perceive that they are the same 

 in the external form, while Mr. Rofe's figures show that the 

 section at d d has the structure of fig. 9, which only differs from 

 fig. 5 & in being straight above instead of concave, and in being 

 divided into two parts. This division is the result of the posi- 

 tion of the arm which cuts the hydrospire in two, in a direction 

 parellel to the fissures. By drawing the points d a and a d 

 together, we get fig. 10, which 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 hydrospires, almost entirely exposed, as in fig. 8, through 

 others in which they are crowded more and more uader th* 

 arms, until at length they become altogether internal. 



Fig. 8. Summit of C. acutus McCoy, m v mouth and vent ; d d suture 

 across the posterior hydrospire. 9. Section across the hydrospire 

 from d to d, at a is the place of the arm. 10. The section con- 

 tracted, as in fig. 6. 11. Summit of Pentremites caryophyllatus De 

 Koninck. 



In C. acutus, fig. 8, only a small portion of the hydrospire is 

 concealed under the arm. In G. Canadensis, a new species, 

 lately discovered in the shales of the Hamilton group in Canada 

 West, each of the four interradial spaces, in which the hydro- 

 spires are placed, is excavated in such a manner as to form a small 

 triangular pyramid, with two of its faces sloping down toward 

 the sides of the two adjacent arms. On these two slopes are 

 placed the hydrospires, which appear to have one fissure entirely 



