. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. 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 th
. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. 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 ^^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
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