92 



genus) I find no reason, especially now lliat it has been shown lo contain a much 

 larger number of species than the genus Temnopleurus itself. 



„Pleurechinus' variabilis Doderlein differs very markedly from all the Pleur- 

 ec/iinus-species mentioned or described above in having in the interambulacra two 

 large shallow depressions along each horizontal suture besides the usual ones at 

 the inner and outer end of the suture, which are, however, only low depressions, 

 not deep pits as in the other P/earec/ifnus-species ; in the ambulacra there is one 

 such intermediate groove. (Comp, the description given by Doderlein : Seeigel von 

 Japan u. den Liu-Kiu-Inseln. p. 18.) This species in fact completely agrees with 

 the genus Opechiims Desor '), and there can certainly be no doubt that it must be 

 referred to that genus. Opechinus is most nearly related to the genus Temnechimis 

 Forbes, both being distinguished by having only low depressions in the sutures, 

 but no true, deep pits, undermining the plates, as are found in Pleiirechinus, 

 Temnopleurus, Salmacis etc. To Duncan is due the merit of having pointed 

 out this important ditTerence'-'). The „P/." variabilis is thus really very different 

 from Pleurechinus. To be sure it is said in Dödehlein's description of this species 

 that the grooves at the end of the sutures „gewöhnlich sehr seicht, manchmal auch 

 scharf und tief erscheinen" (p. 19), an assertion which is in direct opposition to 

 the primary importance of the difference between true and false pits. But Dödeu- 

 lein is wrong here, having confounded specimens of Pleurechinus variegatus with 

 the true variabilis (which has evidently also caused the name variabilis), as I can 

 state, Prof. Doderlein having most kindly placed his material at my disposal. 

 The specimen figured by Doderlein (Taf. LXI. Fig. 5 a — b) as Pleurech. variabilis }u\. 

 is really Pleurech. variegatus. 



Duncan (On the genus Pleurechinus. p. 449) makes Opechinus a synonym of 

 Temnechimis Forb., „as its essential character, never generic, is due to chances of 

 growth of ornamentation". For this assertion there is no reliable evidence. In 

 Duncan & Sladen's Monograph of the Tertiary Echinoidea of Kachh and Kattywar 

 (Palæontologia Indica. Ser. XIV. 1883) it is stated for Temnechinus {Opechinus) Rousseaui 

 that „the fossettes are not seen in the very young form ; and in some large speci- 

 mens there is so little trace of them that they resemble species of Salmacis^ and 

 the plates are then not bevelled" (p. 55). But there is no proof at all that these 

 specimens are really the same species as those with the fossettes well developed; 

 this cannot be made out with certainty in fossil specimens, where the characters 

 found in pedicellariæ and spines etc. have been lost. I quite agree with Gregory 

 (British Fossil cænozoic Echinoidea. Proc. Geologists Assoc. XII. 1891. p. 29) that 

 Opechinus must be maintained as a distinct genus besides Temnechinus, the Crag- 



') In Rev. of Ech. p. 465 Agassiz says the same of Pleurechinus boihryoides, which would have 

 been true if his description of that species had been correct. But PI. bothryoides has really onlj' two 

 pits in each horizontal suture, and it does not correspond to Opechinus. 



') On some Points in the Morphology of the test of the Temnopleuridæ. J. Linn. Soc. XVI. 1881. 



