438 THE CANADIAN NATURALIST. [Dec. 



The genus Leskia is described, in 1851, by Dr. J. E. Gray, in 

 the " Annals," and subsequently, in 1855, in the Catalogue of 

 Recent Echinida, from specimens from Lugard, in Mr. 

 Cummings's collection. It is most intimately allied to the 

 Spatangidae, of which it has the general stamp, but is distinguish- 

 ed from them, and therefore the type of a peculiar family 

 (Leskiadce Gray) or tribe {Palcvostomata Loven) by the peristome 

 and periproct being closed up with a few " triangular converging 

 valves," those of the vent with some small " spicula" in the centre. 

 Dr. Gray has already remarked that " in the form of the mouth 

 and vent it has considerable affinity with the fossil Cystidea, 

 especially the genus Echinosphserites." The detailed description 

 given by Prof. Love'n quite confirms this remarkable combination 

 of features ; the characters assigned to the Palasostomata are 

 as follows: " testa oviformis, peristomium non labiatum, pentago- 

 num, cequilaterale, ore quinqueralis, anus intra periproctium 

 centralis, valvis clausur quinque octo ; aperturce genitales Linos ; 

 semita unica peripetalaT Leskia is a true Spatangoid, save the 

 mouth and the vent ; the latter, instead of being surrounded by a 

 threefold circle of minute plates, the greater and outermost, has only 

 five, seven, or eight great triangular outer plates, and an equal num- 

 ber of minute inner papilla?. The peristome is not bilabiate with 

 a prominent under-lip, nor is it formed principally by the ambula- 

 cral plates ; it is pentagonal, and bordered almost exclusively by 

 the interambulacralia ; there is no buccal membrane covered with 

 three to five series of irregular plates, decreasing inwards, but 

 the mouth is closed up by five equal triangular plates, inserted on 

 the five sides of the peristome. " No living Echinid has such a 

 mouth;" but the author thinks that the genus Toxaster of the 

 1 Neocomien Inferieur,' whose peristome was pentangular, not 

 labiate, might possibly — though the configuration of its mouth 

 somewhat more approaches to that of the true Spatangidae — have 

 had a similar organization. 



In the Silurian Cystidea again, we find precisely the same 

 structure as in the recent East Indian Sea-urchin, viz., in the 

 commonly so-termed c ovarian pyramid,' which, after the opinions 

 of Gyllenhal, Wahlenberg, Pander, Hisinger, de Koninck, and 

 Billings, is really the mouth, whilst Von Buch, with some incon- 

 sistence, makes it the mouth of Caryocrinus, but the genital outlet 

 in the other Cystidea, and Joh. Muller and Volborth sought the 

 mouth in the centre of the converging ambulacral furrows. The 



