101 



Mr. Lyon alludes is seen above, in fig. 58 just below the letter h . It is 

 the same 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 con- 

 cealed under the arm. The five points would then be situated in the 

 angles between the five ambulacra as they are in the genus Fentremites, 

 fig. 63. The concealment of the hydrospires may also be the result of the 

 widening of the arm. This is well shown in P. caryophyllitas De Kon- 

 inck, (P. Orhignyanus 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. 58, 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 betw^een 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 probably institute a new 

 genus for their reception. 



Our specimens of C. Canadensis are well preserved and show the 

 characters of the arms perfectly. After many careful examinations 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 pores in Cadaster are not pores, 

 but the small pits or sock'ets which received the bases of the pinnulae. 

 The rays therefore in this genus are not " pseudambulacral fields," in the 

 sense in which that term is used in descriptions of species of Fentremites, 

 but simply recumbent arms, identical in structure with those of the cysti- 

 dean genera Glyptocystites, Callocystltes, Apiocystites, and others. They 

 lie upon the surface of the plates which constitute the shell of the animals, 

 not imbedded into them as in Fentremites. The large lateral aperture is 

 both mouth and vent, and the central opening heretofore called the 

 mouth is the ambulacral or more properly, the ovarian orifice. As, there- 

 fore. Codaster has the arms of Aoiocy stites, the hydrospires of Fleuro- 

 cystites and the confluent mouth and vent common to all Cystideans, 1 

 propose to remove it from the Blastoidea and place it in the order Cys- 

 tide a. 



4. On the genus Fentremites. 



In Fentremites the hydrospire is an elongated, internal sack, one side 

 of w^hich is attached to the inside of the shell, while the side opposite, or 



