W. KEEPING ON THE GENUS PEL AN ECHINUS. 929 



fectly calcified test, and its free pore-plates ; but I would link them 

 close in classification. It is this resemblance to a notoriously flexible 

 "Urchin which confirms my impression of the pliability of Pelane- 

 chinus — an impression at once suggested by the general appearance 

 of the Cambridge specimen, which, although compressed, is not 

 fractured, but has an undulating surface as though it had of itself 

 collapsed. Commonly, when an Echinus is crushed its brittle plates 

 are fractured across ; but in this not a single plate is broken. Again, 

 the extreme thinness of the plates themselves induces the belief that 

 Pelanechinus was an exception to the rule of rigidity in the Sea- 

 urchins. 



Amongst the Cidaridse, Dorocidaris papillata *, and Gidaris coro- 

 nata as described by Quenstedtf, present the same essential features 

 in their mouth-membrane plates ; but their coronal characters are 

 very different from those of Pelanechinus. 



Although this Sea-urchin was referred to the genus Hemipedina 

 by Dr. Wright, these latter specimens have shown that there is no 

 real affinity with that genus ; and, moreover, its test is distin- 

 guished from all species of Hemipedina by the trigeminal-pore 

 arrangement J. 



The extreme narrowness of the plates will be useful in distin- 

 guishing fragments from Pedince and Hemipedina;. 



Professor A. Agassiz has, in his ' Revision of the Echini,' already 

 pointed out the similarity of the imbricating mouth-plates of Asthe- 

 nosoma to the whole test in several genera of our most ancient 

 (Palaeozoic) Echini ; and he also suggests that the coronal system 

 of the latter may have been altogether suppressed. In this inquiry 

 Pelanechinus is likely to prove very instructive when more speci- 

 mens are obtained, such as will settle the nature of the thin irre- 

 gular plates described supra, p. 927. 



It must not, however, be supposed that there is any near rela- 

 tionship between these modern Echinothuridse and the ancient 

 Perischoechinidae. They are well separated by important and 

 fundamental structural differences, and they form two separate 

 branches distinct from their origin. The elder group is probably 

 the highway from the Cystids to the Echinoids, &c. The Echino- 

 thuridae are a branch from the latter. 



Pelanechinus is a link between the ordinary Diademoid and the 

 more exaggerated Echinoihuria and Asthenosoma. 



Only this one species of Pelanechinus is yet known (viz. P. coral- 

 linus, Wright, sp.) ; but there are other oolitic Diademoid Urchins, 

 such as Hemipedina marchamensis, Wright, which may prove to 

 belong to our new genus. Especially, too, I would notice a Chalk 

 species of Diadema with thin and narrow plates and hollow spines, 

 of which I have seen some fragments. 



* A. Agassiz, ' Revision of the Echini,' t. ii. b. f. 2 & 3. 



t Quenstedt, ' Petrefactenkunde Deutschlands,' t. lxii. f. 100. 



J It is true that H. Tomesii has trigeminal pores ; but these groups of pores 

 are abnormally directed upwards and outwards, whilst Pelanechinus has the 

 usual arrangement in its trigeminal series. 



Q. J. G. S. No. 136. 3 q 



