148 Mr. E. Billings on the Structure of 



manner that two of them abut against the outer extremity of 

 each of the arabulacral ossicles, and extend outward towards 

 the interradials. This seems to prove that the marginal plates 

 belong to the ambulacra, as pointed out by Mr. Lyon, and not 

 to the interradials, as represented by other authors. Although 

 I have studied a large nmnber of specimens, none of them 

 were sufficiently perfect to enable me to make out the whole 

 structure of this part of the test of Nucleocn'nus. I have, how- 

 ever, seen enough to convince me that the ambulacra are much 

 more complex than is usually supposed. The lancet plate, if 

 it occur at all in this genus, must be very narrow. The am- 

 bulacral groove, as in Petitremites, sends off branches right 

 and left. There is also evidence of the existence of minute 

 marginal plates on each side of the groove. 



The hydrospires are ten elongated sacs, each with two deep 

 folds. They are perfectly homologous 

 with those of Fentremites, only differing ^^o- 7- 



therefrom in not being united in pairs ; 

 consequently there are ten spiracles in- 

 stead of five. The mouth, or oro-anal 

 orifice, is larger in proportion to the size 

 size of the body than it is in Pentremites. 

 Mr. Meek informs me that the moutli in 

 some of the Blastoidea is protected by a 



single valve that covered it like the lid [Pljj. 7. Transverse section 



of a jug. From the structure of the '^S^i^!:^^^^ 

 orifice, I am inclined to think that in ^-.ti^e two anterior hydro- 



- ' . . -. • •^ spires; p, pore leading into 



NucieOCrinUS it possessed a similar pro- the hydrosiiire ,- .9, one of the 



. ,• grooves. 



tection. 



In the apex nearly all the space within the circle of aper- 

 tures is covered by a thin integument of small plates (fig. 3). 

 When this is not preserved, a large subpentagonal aperture is 

 seen, as shown in fig. 5. This aperture occupies the position 

 of the mouth in the existing Echinoderms. The integument, 

 as will be shown further on, represents that which covers the 

 mouth of an embryonic starfish. Mr. Conrad described this 

 genus, in 1842, as liaving only one aperture in the summit : — 

 " This genus differs from Pentremites^ Say, in having only one 

 perforation at top, which is central " (Journ. Acad. Nat. Sci. 

 Phil. vol. viii. p. 280, pi. 15. 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 Pentre- 

 mites. He described the apex as being provided with six aper- 

 tures, five of which were divided by a partition within each : 

 these he considered to be the ovarian orifices. The sixth he 



