328 



THE GEOLOGIST. 



The question presented to me by Mr. Flower's fossil was, whether to 

 consider it part of the envelope of a new kind of Holothuria or whether 

 it might be no more than a fragment of the oral disk of some great 

 unknown Echinus. Portions of the imbricating scaly armour of a Psolus 

 had been met with when examining the fossils of the boulder clay collected 

 by Mr. J. Bichmond, of Bothsay ; but in Psolus, while the greater part of 

 the body is clothed with fish-like scales, the ambulacra arc only developed 

 on one side, forming a creeping disk, the scales of which are small and not 

 imbricated. On the other hand, the peristome of the largest known Ecki- 

 nite from the Chalk is less than an inch in diameter ; and the largest 

 recent sea-urchin in the museum has an oral disk not more than 2 inches 

 wide, whereas the fossil is a segment of a disk which must have been 

 at least 4 inches across. This objection, on the score of size, was how- 

 ever less felt, because the Cyphosomas and Diademas of the Chalk have 

 larger oral and apical orifices than any other urchins, and the character 

 of their apical disk was unknown, being only preserved in a few minute 

 specimens of C. difficile, from Chute Parm. Moreover, there were indi- 

 cations in the Upper Chalk of a great Diadema, of which nothing more 

 had been obtained than scattered plates and fragments of spines. This 

 species is referred to in Decade V. of the Geological Survey (Article Dia- 

 dema, Section C, spines tubular, annulated). Mr. "Wetherell obtained a 

 mass of chalk containing above one hundred fragments of spines, which 

 are hollow, striated and annulated, as in the recent _D. calamaria. Prom 

 the plates mingled with the spines we ascertained that the ambulacral pores 

 presented the usual characters, being arranged in single file, and a little 

 crowded near the peristome ; but many of the plates presented only their 

 smooth inner surfaces. A smaller mass of chalk, in Mr. "Wiltshire's 

 cabinet, contains similar plates and spines, mingled with a few true scales 

 and minute truncated spines like those of Echinothuria. The Diadema 

 spines were erroneously referred by Prof. E. Forbes to the genus Micraster 

 (decade iii. pi. 10, fig. 15; bad, for they are not spiral). They are also 

 figured by Dixon, in his ' Geology of Sussex,' and described by Forbes as 

 " spines of a Cidaris." Diademas possessing spines of this character are 

 known to occur in the Upper Cretaceous strata of France ; and Dr. Wright 

 has lately obtained a small specimen from the chloritic marls of Dorset- 

 shire. In these the apical disk is quite small. 



A more serious difficulty, in comparing Mr. Flower's fossil with the oral 

 disk of any Echinite, was presented by the arrangement of the plates ; in 

 the recent Echinidae (like the Cidaris represented by Fig. E) they are all 

 directed towards the dental orifice, but here the alternate series take 

 opposite " dips," the ambulacral plates overlapping one way and the others 

 in a contrary direction. 



Last year, while I was still hesitating about the publication of Mr. 

 Flower's fossil, a second specimen was obtained from Charlton, in Kent, 



