91 



The validity of the genus Pleiirechimis has been somewhat disputed. Agassiz 

 (Rev. of Ech.) regards it as a subgenus of Temnoplearus , and this is also the position 

 accorded to it by Duncan M in his „Revision of the Genera and great Groups of 

 the Echinoidea" (p. 107). Bell (Echinodermata. „Alert" p. 119) makes it synony- 

 mous with Temnoplearus, whereas Döderlein (Echinoidea von Amboina — Semon's 

 Reisen) recognizes the validity of a genus or, at least, subgenus Pleurechinus, 

 distinguished „durch ihre auffallend kugelig gerundete, meist hohe Schale, durch 

 die fehlende Crenulierung ihrer Hauptwarzen, besonders aber dadurch, dass auf der 

 Buccal- wie Apicalseite die Ambulacral- und Interambulacralplatten regelmässige 

 Horizontalreihen kleiner Hauptwarzen tragen, endlich durch ihre sehr kurzen und 

 überall etwa gleich langen Stacheln". — „Die Apicalseite ist ferner bei Pleurechinus 

 der Buccalseite sehr ähnlich, bei Temnoplearus sind beide Seiten sehr verschieden". 

 Through the comparatively large number of species made known in this paper, the 

 characters pointed out by Döderlein as distinctive of the genus are modified to 

 some extent. The form of the test is low in siamensis and scillce, being very dif- 

 ferent from such species as bothryoides and maculatus. no generic character is then 

 found in the form of the test. The primary and secondary tubercles do not form 

 transverse series in Döderleini, siamensis and scillce. This feature must accordingly 

 also be left out of consideration as a generic character. The absence of crenulation 

 is a more general feature, but not quite exclusive, the tubercles being rather 

 distinctly crenulate in larger specimens of variegatus; this is, however, evidently 

 a very important character uniting this group of species. The large pits would be 

 another excellent character, were it not for the two species ruber and variegatus, 

 where they are rather small. It is also to be remarked that there is a covering 

 membrane round the edge of the pits, leaving only a smaller opening in the middle 

 — but this is not found in variegatus (and, probably, not in ruber). The uniform 

 character of the spines and of the two sides of the test holds good. No generic 

 character is found in the pedicellariæ. It is thus rather difficult to give a good 

 definition of the genus Pleurechinus. If the species ruber and variegatus were re- 

 moved from the genus, the large pits bordered by a membrane would give an excel- 

 lent character for the other species; and if Döderleini were removed also, the rest 

 of the species would form a somewhat more close group, though not even then a 

 uniform group, the species bothryoides and maculatus on the one hand, siamensis 

 and scillce on the other hand forming special groups. It seems, however, unneces- 

 sary to make these groups each a separate genus. I prefer to leave them all-in the 

 genus Pleurechinus, which is, I think, sufficiently distinguished from Temnopleurus 

 by its noncrenulate tubercles (in spite of the large specimens of variegatus) the 

 uniform size of the spines and the uniform aspect of both sides of the test. To 

 regard it as a mere subgenus of Temnopleurus (or even synonymous with that 



') Comp, also Duncan's paper: On the genus Pleurechinus, L. Agass., its classificatory position and 

 alliances. J. Linn. Soc. XVI. 1882. p. 447. 



12* 



