1000 DB. J. W. GBEQOBY ON A NEW [DeC. 15, 



4. On Lysechinus, a new Genus of Fossil Echinoderms from 

 the Tyrolese Trias. By J. W. GuegorYj D.Sc, F.G.S., 

 Assistant in the British Museum (Nat. Hist.). 



[Received October 22, 1896.] 

 (Plate LI.) 



I. Introduction 1000 



II. Description o( Lysechinus incongruens, gen. et sp. nov lOOI 



III. Affinities of Lysechiims and Classification of the Plesiocidaroida. 1001 



IV. Affinities of tlie Plesiocidaroida 1003 



I. Introduction. 



The genus Tiarechinus was founded by Neumayr' in 1881 for 

 a fossil from the St. Cassian Trias, which had been previously 

 studied by Laube, whose name, however, had not been published. 

 Neumayr described the fossil as an Echinoid having characters 

 which allied it to the ArchfBocidnridaj, Cidaridao, and DiadematidaB. 

 He included it temporarily in the first-named family, but thought 

 it would probably be necessary to institute for it a new order, 

 intermediate between the Palaeechinoidea and Euechinoidea. The 

 main characters of the genus relied on by its founder were its 

 large apical disc, short ambulacra, large mouth, and its having the 

 granulation uniform, except for four small tubercles at the oral 

 end of each interambulacrum. He thought that he could recognize 

 certain sutures by the use of glycerine, but it was reserved for 

 Loven ^ to prove that each interambulacrum consists of four 

 plates, three vertical plates resting on a single oral plate. This 

 discovery showed that Tiarecldiius was even more abnormal than 

 Neumayr thought. Duncan^, in 1890, accordingly made it the 

 type of a new order, the Plesiocidaroida, in which it has since 

 been allowed to remain in solitai-y state. In the same year I found 

 a specimen in the Klipsteiu Collection in the British Museum, 

 which I at first regarded ns a new species of Tiarechinvs, an 

 opinion which was shared by the late P. H. Carpenter, to whom 

 I showed it ; but a careful examination of the type specimen at 

 Vienna, and of others there and in Berlin, showed that it was a 

 distinct genus having the same type of structure. 



1 M. Neumayr, " Morphologisclie Studien fiber fossile Echinodermen," Sitz. 

 k. Akad. Wiss. Wien, Bd. Ixxxiv. Abt. 1, 1881, pp. 169-17G, pi. ii. fig. 4. 



' S. LoT6n, " On Pourtalesia," Ilaudl. K. Svena. Vet.-Akud. Bd. xix. 1883, 

 no. 7, pp. 12, G."), pi. xiii. 



" I'. M. Duncan. " A Koviaion of tlio Genera and Great Groups of' the 

 Bchiuoidca," Journ. Linn. Soc, Zool. vol. ixiii. 1890, p. 19. 



