82 PANAMTC DEEP SEA ECHINI. 



diameter, but gives no detailed figures of the ambulacra! mens of that 

 stage. 



The figures given by Cotteau' of Tetracidaris do not include the abacti- 

 nal plates of the interambulacral area. The passage between the two pairs 

 of interambulacral plates to the usual type of interambulacral plates is 

 abrupt and gives us no clue of their origin. We may notice that the 

 outer suture of the inner plates is not in the median line, hut in the lower 

 part of the inner plate, the outer plates being pentagonal, the inner plates 

 hexagonal. 



Lysechinus, 2 although it has three rows in two sets above the second row, 

 yet gives no indication of how the third median row has originated any 

 more than Tiarechinus, which has three plates immediately above the 

 primordial. 



After Dr. Mortensen's criticisms 3 of the chapter on the Echinothurise 

 of the "Challenger," it was natural to expect that in the arrangement of 

 his figures we should have a model to follow. It was essential, if he wished 

 to persuade his readers of the value of his identifications, that every effort 

 should be made to ensure ready comparison of the pedicellarire of allied 

 species. Quoting from Dr. Mortensen, " it was almost enough to drive one 

 to despair" to make such an attempt with his plates, without a guide or 

 key to their arrangement. 



In dwelling upon the many points of relationship between Phormo- 

 soma and Asthenosoma I drew attention to the difficulties of describing 

 the species of these genera owing to the changes due to growth. On the 

 strength of this remark Dr. Mortensen assumes that I have stated that the 

 two genera cannot be distinguished, 4 and proceeds to ignore all that has 

 been said of the different species of EchinothuriaB relating to the actinal and 

 abactinal systems and the spines, because he thinks the Echinothurids are 

 not adapted for examination in the dry state. But he claims to give a 

 perfect classification based, first, upon the characters of the spines, as if his 

 predecessors had not mentioned them in any way ; next, upon pedicellari:e. 

 tube feet, pores and spicules, the last of which he has previously informed us 

 were of no systematic value ! Having stated that the genera Phormosoma 



1 Bull. Soc. Geol. de France, 3« Ser. I, PI. Ill, p. 258. March 17, 1873. 



- On Lysechinus, a new genus of fossil Kchinoderms from the Tyrolese Trias. J. W. (hegory, 

 Proc. Zobl. Soc. London, Dec. 15, 1896, p. 1000, PI. LI. 

 8 Mortensen, 1. c. p. 43. 

 4 Mortensen, 1. c. p 13. 



