1882. | PROF. F. J. BELL 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 Psolus 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 
Polian vesicle, the size of which in P. faéricii, 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 priori 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 an 
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 covering 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, Lissothuria®, where “the upper 
surface of the body is covered with a soft smooth skin, in which are 
imbedded minute perforated plates.” 
1 See Verrill, Proc. Bost. Soc. Nat. Hist. x. p- 353. 
2 Trans. Conn, Acad. i. p. 822. 
