1870.] BILLINGS — ON CRINOIDEA AND BLASTOIDEA. 187 



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 number of specimens, none of them were 

 sufficiently perfect to enable me to make out the whole structure 

 of this part of the test of Nucleocrinus. I have, however, seen 

 enough to convince me that the ambulacra are much more com- 

 plex than is usually supposed. The lancet plate, if it occur at all 

 in this genus, must be very narrow. The ambulacral groove, as 

 in Pentreniites, 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 

 sacks, each with two deep folds. They 

 are perfectly homologous with those of 

 Pentremites, only differing therefrom in 

 not being united in pairs ; consequently 

 there are ten spiracles instead of five. 

 The mouth, or oro-anal orifice, is larger 

 in proportion to the size of the body 



Fig. 7. Transverse section ,•, • • • • n ^ -^ h,t -^^ i 



through a specimen which hits than it IS in Fentremites. Mr. Meek 



all the hs'drospires preserved. • n ai, i. j.-l xi • r> 



h, the two ariteripr hydro- mtorms me that the mouth in some of 

 hf^rli&rrtfni'Ttl the Blastoidea is protected by a single 

 ^'^^''''- valve that covered it like the lid of a 



jug. From the structure of the orifice, I am inclined to think 

 that in Nucleocrinus it possessed a similar protection. 



In the apex, nearly all the space within the circle of apertures 

 is covered by a thin integument of small plates, fig. 3. When 

 this is not preserved, a large snb- pentagonal 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 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 



