117 



In the apex, nearly all the space ^Yithin the circle of apertures is 

 'Covered by a thin integument of small plates, fig. 73 When this is not pre- 

 served, a large sub-pentagonal aperture is seen, as shown in fig. 75 This 

 aperture occupies the position of the mouth in the existing echinoderms. 

 The integument, as will be shown further on, represents thatwhich covers 

 the mouth of an embryoiic Star-fish. Mr. Conrad described this genus 

 in 1842, as having only one aperture in the summit. " This genus differs 

 from Pentremites, Say, in having only one perforation at top, which is 

 central." (Jour. Acad. Nat. Sci. Phil, vol. viii, p. 280, pi. xv, fig. 17). 

 His figure represents the fossil with the apex downward. Dr. Ferd. 

 Roemer, showed that, when perfect, there is no central opening, and he 

 made this one of the grounds for separating the genus from Pentremites. 

 He described the apex as being provided with six apertures, five of which 

 were divided by a partition within each. These he considered to be the 

 ovarian orifices. The sixth he supposes to be both mouth and vent, which 

 accords with my view- (Mon. der Blastoiden, p. 378). In 1868 I dis- 

 covered the five small pores at the apical extremities of the ambulacral 

 grooves. In general it is difficult to see these pores, but if a sihci- 

 fied specimen, which has been fossifized in a calcareous matrix, be placed 

 in an acid for two or three minutes, the acid cleans them out and 

 they then become distinctly visible. I believe these to be the pores through 

 which the ovarian tubes passed outward along the grooves to the pinnulie. 

 There are thus, sixteen apertures in the apex of Nudeocrhius, — ten spira. 

 cles, five ovarian orifices, and one oro-anal aperture. There are no true 

 radial plates. The whole of the test with the exception, perhaps, of the 

 ambulacra belongs to the perisomatic system. 



8. On the occurrence of Enibrtjonic forms among the Palceozoic Echinc- 



derms. 



No proposition m Natural History has been more clearly demonstrated 

 than this: — That, in general, the paleozoic animals resemble, both in external 

 form and internal structure the embryonic stages of those of the same class at 

 present existing. Prof. Agassiz has long taught in his lectures and various 

 publications, that this is especially observable in the Echinodermata. Judg- 

 ng from the figures and descriptions of MuUer, Agassiz, Thomson, Carpenter 

 and others, I should say, that in this class, the most striking resemblance is 

 that which occurs between the adult stages of the Cystidea, Blastoidea, and 

 Crinoidea, on the one hand, and the embryonic Star-fishes on the other. 

 The structural character that has the most important bearing on the 

 subjects discussed in these notes, is, that in all four of these groups, the 



