34 ECHINODERMS OF THE LONDON CLAY. 



not thicken or swell out at the point of junction. The five plates of the pelvis alternate 

 with five large, slightly convex, ascending primary radials, which bear the first arm-plates 

 large also, but of only half the height. All the plates are smooth. The arms, &c, are 

 destroyed. The column is uncompressed, very equal in its proportions, very smooth, with 

 slightly undulating joints, whose sutures appear slightly curved externally, and. whose 

 articular surfaces are roughly radiated. The perforation is very small. The whorls of 

 ramules are very distant ; their sockets are large and deeply impressed. They are 

 slender, but strongly jointed. 



Genus — Pentacrinus, Miller. 



Cup very shallow, constituted of a pelvis composed of a single piece formed out of 

 five anchylosed plates, alternating with five primary radials. Column more or less 

 distinctly pentagonal ; furnished with articulated ramules. Joints with stellated articular 

 surfaces. 



1. Pentacrinus subbasaltieormis. Plate IV, figs. 8, 9, 10. 



Pentacrinus stjbbasaltifokmis, Miller. Nat. Hist, of Crinoidea, p. 140. 



— — Wetherell. Trans. Geol. Soc, London, 2d series, 



vol. v, pt. 1, p. 136, pi. viii, fig. 4. 



— — Austen. Monog. Rec. and Fos. Crinoidea, p. 122, 



pi. xvi, fig. 2. 



— didactylus, Auide D' Orbigny. Mem. Soc. Geol. France, 2 de ser. vol. ii, 



pi. v, fig. 18? 



Miller, in his famous work upon Crinoidea, proposes the name of Pentacrinites 

 subbasaltiformis for the columns of a Crinoid, found by Mr. James Sowerby in the 

 London Clay at White Conduit House, Islington, and mentions that similar columns occur 

 at Richmond and at Kensington. He remarks that " these columns much resemble in 

 size and shape those of Pentacrinites basaltiformis, but have the angles more rounded. 

 From their exhibiting no marks of muscular corrugation at their exterior surface, and the 

 joints being of uniform thickness, I apprehend the fragments before me to be full grown 

 columnar portions." It was figured by Mr. Wetherell in the illustrations to his paper 

 entitled " Observations on a Well dug at Hampstead Heath," and since by Mr. Austin, 

 in his " Monograph of Recent and Fossil Crinoidsea." 



Numerous fragments of stems have been found. These vary from round to very obtusely 

 pentangular, and from five lined to five grooved along this length. The joints arc of equal 

 dimensions, and are plane and quite smooth externally. The articular surfaces present 

 rounded crenated lobes. At intervals, ramules are given off opposite, or very nearly oppo- 

 site, each other, disturbing the symmetry of the joints from which they spring. In the 

 example represented, Plate IV, fig. 8, the diameter of the joints is one fourth of an inch, 

 and their altitude one tenth of an inch. 



