93 



species and the Oriental species forming two very well defined groups, the one 

 without, the other with intermediate depressions. „The differences between the 

 two groups of species may be only due to the disposition of the raised epistromal 

 ribs, but these affect so materially the whole aspect of the tests, and the epistroma 

 plays so prominent a part in the classification of the Glyphocyphinæ that it is con- 

 venient to express the differences in this way". „P/." variabilis is then a recent 

 representative of the genus Opechinus, so well represented in the Tertiary of India 

 by the species: Rousseaui d'Arch., Hooked d'Arch., costatus d'Arch., tuberculosus 

 d'Arch., Valenciennesi d'Arch. (this is the first named but, unfortunately, the least 

 typical species, the intermediate grooves not being distinctly separated from the 

 angular ones), percultus Desor, affinis Dune. & Slad. (but not Temnech. stellulatus 

 Dune. & Slad. nor Gajensis Dune. & Slad., which have no suturai depressions, but 

 a strongly reticulated surface; they are probably nearly related to Genocidaris and 

 Trigonocidaris). 



PoMEL (Op. cit. p. 85) makes the species costatus d'Arch. the type of the 

 genus Opechinus, referring the species Valenciennesi, Rousseaui, Hookeri, tuberculosus 

 and percultus to the genus Pleurechinus. — This wrong representation of the two 

 genera evidently is caused by the fault in the description of PI. bothryoides of 

 Agassiz pointed out above. To separate the species costatus from the other sjiecies 

 with intermediate fossettes on account only of its having a depression in the middle 

 of the interambulacral plate in succession to the series of depressions along the oppo- 

 site suture seems rather absurd, the more so as in other species the median suturai 

 depression may proceed on the median point of the opposite interambulacral plate 

 (in 0. spectabilis f. i.). 



It is interesting to note the assertion of Desoh with regard to this genus 

 (Synopsis des Echinides fossiles, p. 107) that „il en existe des espèces vivantes"; it 

 thus seems that he has seen specimens of some recent form of Opechinus; likewise 

 the de.scription of PL bothryoides in „Rev. of Ech." might seem to indicate, that 

 Agassiz has seen such a specimen (comp, above p. 83). In the „Challenger"- 

 Echinoidea (p. 108) Agassiz further says of some small specimens of Pleurechinus 

 bothryoides (those specimens, 1 suppose, which are not bothryoides but, probably, 

 variegatus): „they show clearly that we may expect to find in the China Seas 

 a species of Temnopleurus still retaining the principal features so characteristic 

 of some of the nummulitic species of India , figured by d'Archiac and Haime" 

 (Temnopleurus Valenciennesi). I am unable to understand how these specimen&.can 

 show this, since all the „Challenger"-specimens of PI. bothryoides are, in any case, 

 true Pleurechinus-species with only two pits in each suture. Hut it is a very 

 curious thing too that among the „Challenger"-Echinids Agassiz really had a 

 species of that interesting group, and even a large, beautiful and well preserved 

 specimen; but he cannot have examined it very exactly, having determined it as 

 Temnopleurus Hardwickii (St. 192). Through the kindness of Prof. Bell I have been 



