1882.] PROF. F. J. BELI, ON THE GENUS PSOLUS. 649 



the test. The differences do not seem to me to be of really generic 

 value, any more than are the distinctions which some have seen 

 between what have been called Psoitis and Cuvieria, or Psolus and 

 Lophothuria, no naturalist, so far as I know, having followed Bronn 

 (Classen u. Ordn. i. p. 404) in the use of the term Lepidopsolus. 



A naturalist need know no other species than P. fabricii and 

 P. phantapus to see what are the kind of claims for generic sepa- 

 ration. In the one case there is a heavily-armed test, formed of 

 strong imbricating scales, with only the margin of the foot provided 

 with sucking-feet, and with the tentacles richly branched ; in the 

 other there are granulations, less richly branched tentacles, and a 

 median set of sucking-feet'. 



An investigation of the internal anatomy will not, however, reveal 

 a difference in the part which should especially be affected in the 

 more firmly bodied forms. We might, that is, expect to find 

 valuable distinctive marks in the grade of development of the 

 Poliau vesicle, the size of which in P. fabricii, or any other heavily- 

 armed form, would be easily enough ascribed to the fact that the 

 impossibility of the walls of their body aiding in the propulsion 

 of fluid through the ambulacral canals would require the propelling 

 organ to be of larger size, and doubtless also of greater proportional 

 strength. A jjriori considerations of this kind are often shown by 

 the dry light of dissection and observation to be as little in conso- 

 nance with fact as the nature of things allows ; and that is cer- 

 tainly the case here : the Polian vesicle of Psolus regalis is propor- 

 tionally as large as, even if it be not larger than, that of P. fabricii. 

 A fact of this kind does, at the same time, teach us that what is 

 apparently an external difference of great importance may be so as 

 between, say, Psolus and Holothuria, but is not a great one between 

 Psolus and Lophothuria. Such being the case, we have here au 

 example of affinities so peculiar that what very rarely obtains 

 among Echinoderms, at any rate, does seem to be presented here — a 

 relationship that can best be indicated in the language of systematic 

 zoology by making use of subgeneric divisions. 



While Psolus may be spoken of as a Gasteropodous dendrochiro- 

 tous Holothurian, with a flattened trivium and the bivium without 

 suckers, and invested in a firm coveriug of calcareous pieces, Psolus 

 (Eupsolus)s. str. will have granular plates, a median row of trivial 

 suckers, and no basal web to the tentacles; Lophothuria large 

 granulated scales or plates, no median row of suckers, and a basal 

 web to the more richly branched tentacles ; while Hypopsolus has 

 the scales invested in a thick integument, and the trivial suckers 

 numerously developed. 



It is possible that future investigation will justify us in associating 

 with these, as another subgenus, imoM una *, where "the upper 

 surface of the body is covered with a soft smooth skin, in which are 

 imbedded minute perforated plates." 



1 See Yerrill, Proc. Bost. Soc. Nat. Hist. x. p. 353. 

 » Trans. Conn. Acad. i. p. 322. 



