260 Prof. De Koninck. — New British Echlnoderms. 



them completely. I should, however, observe that the genital plates 

 of PalcBcMnus differ essentially from those of all the Echinodermata 

 belonging to the more recent geological period, by the presence of 

 three well-marked pores on four of them, whilst the fifth is usually 

 perforated by only one aperture. 



I shall avail myself of this occasion to rectify a determination that 

 I made in 1844. In my work on the Carboniferous fossils of 

 Belgium, I have described and figured, under the name of Cidaris 

 Munsterianus, a few plates and radicles of a species of Echinus that I 

 discovered at Vise, and which M. Desor referred in 1858 to his genus 

 Eocidaris,^ but which in reality appertains to the genus Lepidocentrus, 

 created by Mtlller in 1856, upon a species very nearly related to 

 mine that he had discovered in the Eifel, and of which he has 

 obtained a fairly complete specimen. The Carboniferous species 

 must, then, be placed under the genus Lepidocentrus Munsterianus. 

 It shows, at the same time, that the species of that genus are not 

 exclusively Devonian, as it was formerly supposed.^ 



The second form to which I would draw attention is a fossil of 

 most peculiar structure, which I had the opportunity to study in 

 Mr. John Gray's collection. It belongs to the family of the Oystide^, 

 of which it constitutes one of the most extraordinary types ; and, 

 from its very peculiar form, it cannot be referred to any known 

 genus of that large family. I am obliged to form a new genus — 

 Placocystites — for its reception. The essential characters are as 

 follows : — Calyx of a compressed form, concave on one side and 

 convex on the other, composed of four or five series of plates, sym- 

 metrically disposed on the sides of the vertical axis, in the line of 

 which is placed the anal aperture, formed by semicircular hollows 

 on the borders of two symmetrical plates placed in juxtaposition 

 above the ovarian plate ; peduncle with an oval suture. I have 

 named the only species of this genus known to me, Placocystites 

 Forhesianus, de Kon. 



Mr. J. Gray possesses two specimens of this genus ; ^ that which 

 I have drawn is the more complete and the best preserved. Both 

 are from the Wenlock Limestone of Dudley, a member of the 

 Upper Silurian series of Sir Eoderick I. Murchison. This species, 

 of middle size, is formed of a base, which seemed to me to be 

 composed of one single plate. On the anterior side are seen 

 nineteen small polygonal plates, not including the base. The two 

 lateral parts are perfectly symmetrical. The base supports a 

 horizontal series of five plates, of which the hexagonal median one 

 is longer than broad, and bears, a little to the left of its centre, a 

 small aperture or ovarial impression, of a circular and well-marked 

 form. The two lateral plates which join these directly are about 

 of equal length, whilst the other two, further from the centre, are 

 wider and longer, each of these last surmounted directly by two 

 others, of which one is very long, and is followed by a shorter one. 



1 P. 35, pi. E, fig. 2, a, b, c, d. 



* Synopsis des E'chinides fossiles, p. 156. 



3 Now in the British Museum. 



