1896.J GENtrS OF FOSSIL BOmNODEBMS. 1005 



lesf, as in tho Plesiocidaroida. In the second case strength was 

 obtiiined by the developmeut of a plate in the centre of the apical 

 system, as in the Saleniidaa, which first appear in the St. Cassian 

 beds. 



The last point it is necessary to consider is from what possible 

 ancestor the Plesiocidaroida may have been derived. I am not 

 aware that any sugpestion has ever been made as to the ancestry 

 of TiarecJiinics. As Jackson remarks, in all echinids after 

 Botliriocidaris there are only two plates in the second row of 

 interambulacral plates, except in Tiarecliinus, where there are 

 three, an arrangement which is " therefore to be looked at as a 

 feature standing quite by itself as a structural detail " '. LysecMwus, 

 however, bridges the gap in this respect between TiarecJiinus and 

 the Palaeozoic echinids. All those typical genera of the latter, in 

 which none of the interambulacral plates pass on to the peristomal 

 membrane, have the oral ends of the interambulacra arranged as 

 in Lysechinus. In them a single peristomal plate is succeeded 

 by two plates, above which occurs a line of three. Lyseeliinus is 

 therefore the more primitive genus. The interambulacra of 

 TiarecJiinus can easily have been produced from it by the resorption 

 of the second zone of interambulacral plates and increase in 

 height of those of the third zone, so that they are left directly 

 superposed on the single peristomal plate. 



The St. Cassian fauna is rich in new types of structure, which 

 probably arose from the somewhat wild attempts of its members 

 to adapt themselves to unfavourable conditions of life. Hence it 

 appears more reasonable to regard the Plesiocidaroida as a random 

 offshoot rather than as an ancestral group, and as being of interest 

 as a biological backwater out of the main stream of echinid 

 development, instead of being its primary source. 



EXPLANATION OF PLATE LI. 



Fig. \a, b, & c. Lysechinus incongrucns, from the Trias of St. Cassian. The 

 test seen respecliTely from aboTe, from below, and from the side. 

 X 4 diani. 



2 a, b, & c. The same in outline ; diagrammatic. 



3 a, b, & c. Tiarcchinns princcps, seen from the same aspects. X 6 diam. 



(After Neumajr.) 



4 a, b, & c. The same in outline ; diagrammatic. (After Lov^n.) 



5. Diagram of an interambidacrum and genital plate of Tiwrechinus. 



6. „ of the same in Lysechinus. 



.. ' R. T. .Tncltson, " Studies of Paliceehinoidcn," Bull. Qeol. Soc. Amcr. vol. vii, 

 18!)6, ].. 243. 



