1044 The American Naturalist. [December, 
oplax, a Gonoplacid (?) crab of a remarkably antique facies, which 
appears to be closely connected also with Cymopolia.” * 
The description and figures appear to me to indicate that the new 
crab has no close relationships with either the Gonoplacids or Cymo- 
polia. 
Through the kindness of Miss Rathbun, of the Invertebrate de- 
partment of the U. S. National Museum, I have been able to study 
specimens of all types and compared them with the data respecting 
Archeoplax, and could find no special features of agreement. Archa- 
oplaz,it seems to me, must be considered entirely independently of 
the types with which it has been contrasted. 
I may preface the further remarks I have to make with the statement 
that the crab so called by Messrs. Alcock and Anderson cannot retain 
the name given to it by them— Archwoplax—as precisely the same form 
had been bestowed more than 30 years ago on an extinct genus, also 
of the superfamily of Grapsoidea, represented by fossils from Gay 
Head, Mass. Archaoplax signifera was the name given by W. Stimp- 
son to miocene tertiary remains found there, and described in the Boston 
Journal of Nat. Hist. (vol. 7, p. 584, 1863). 
As a new name is therefore necessary, I would suggest as eminently 
appropriate for the crab made known by Messrs. Alcock and Anderson, 
the generic designation Retropluwma (retro, back or backward, and 
pluma, a soft feather). The applicability will become evident in due 
course 
When I first saw the figure of the mouth parts I inferred that the 
external pair of maxillipeds had been lost, but Messrs. Aleock and 
Anderson expressly declare (p. 182) that “the external maxillipeds 
are so small and slender as to leave completely exposed the mandibles, 
the wide endostome, and a part of the wide and produced efferent 
branchial channels.” They give the figures as those of a perfect 
animal, and apparently had a number of specimens. We are, there- 
fore, placed in the dilemma of assuming that the crab differs radically 
from all others, or that the learned authors may have been mistaken ; 
I prefer, in this dilemma, to leave the question open for re-examination 
by the original describers. 
The new type, however, differs in another character almost as 
remarkable as would be such an extreme and anomalous modification 
of the maxillipeds supposed by its describers. 
* It is later (p. cd suggested that “its nearer affinities are, perhaps, with the 
Macrophthalmines. 
+“ Bay of Bengal, at almost all stations off the Coromandel coast, from 140 
NUMOS, between 100 and 250 fms.” P. 183. 
