2 



BRITISH FOSSILS, 



by the name of Cyphosoma. None of these are yet figured in these 

 Decades.* They differ essentially from Diadema, by having the 

 tubercles imperforate, as it is called, or without a central de- 

 pression ; and as a character of habit may be distinguished by 

 having the tubercles on the ambulacral area as large as those of the 

 interambulacral, while in Diadema the latter are almost always of 

 greater size. The present genus ranges through the whole of the 

 secondary formations up to the present time ; the Gyphosoma is 

 confined to the chalk formations. 



The numerous species of Diadema described by Agassiz and 

 other authors fall, according to the Swiss naturalist, under three 

 groups or subgenera, the characters of which are given in the 



Catalogue Raisonne/' In the first group, of which the living 

 D. euTopwum is considered the type, the interambulacra] areeo 

 have but Uuo rows of primary tubercles, without secondary rows. 

 Of this type, D. depressum, Ag., and D. mamillanum, Roemer, are 

 British fossil examples, and the majority of the species in the genus 

 are at present arranged in it. Of the second group the species here 

 figured is a good example. It has, besides the two primary rows of 

 tubercles, secondary rows of less size. In the third group (Tetra- 

 gramma), the multiplication of the larger tubercles has proceeded 

 still farther, there being four rows at least on each interambulacral 

 space. D. Brongniartii, a cretaceous fossil, is a good British species 

 of the type.i" 



Description. — Diameter, 2 inches 5 lines ; height, 1 inch 8 lines ; 

 diameter of the mouth (exclusive of the notches), 11 lines. General 

 form, subhemispheric ; the upper surface depressed above, the infe- 

 rior surface very much flattened. From the depressed apex the 

 sides slope a little outwards to the rounded margin, radiated pretty 

 equally by the five lanceolate ambulacra, which are very little, if at 

 all, more prominent than the intervening spaces. They are fur- 

 nished with two rows of primary tubercles, about twenty in a row, 

 which are small near the apex, and increase in size towards the 

 margin, where three or four become abruptly larger ; all are per- 

 forate, and on crenulated bosses. Between them, for all the lower 

 two thirds of the ambulacrum, there is a single row of small 



* A notice of some species of this genus, considered ne^ by the late Prof. E. Forbes, 

 will be found at the end of this Decade. 



f Desor combines (in his recent work, 1855) the first and second of these groups in 

 the section " Pseudodiadema,'" and he also separates those species which have the pores 

 crowded above (Z>. suhangulare, Agass., and D. pentagonum), under M'Coy's genus 

 Diplopodia. (See Appendix.) 



