1g16.] S. Kemp: Notes on Crustacea Decapoda. 383 
Groups I and V of Pontophilus. Ortmann! in 1890 published a 
genealogical tree of the five genera known to him; but the evidence 
afforded by the pleopods leads to results directly at variance with 
those which he obtained and it will be well therefore to discuss the 
matter briefly. 
The two most primitive groups of species in the family are in 
all probability Aegeon and Pontophilus, Group I. They possess— 
most of them at any rate—a rudimentary exopod on the first legs, 
the number of branchiae is at least as great as that in any other 
genus and the endopod of the last four pairs of pleopods is well 
developed and possesses an appendix interna. In Aegeon the 
second leg is larger than in Pontophilus, Group I, agreeing more 
nearly with that of Group V, and the former genus also possesses 
a rather larger number of branchiae than the latter. In this last 
feature it appears to be the more primitive of the two, but in 
the curious C-shaped form of its gills it presents a character not 
known in any other genus of Caridea. Aegeon, in my opinion, 
must be regarded as an offshoot of the original stock from which 
the other genera are descended. 
The evolution of the remaining genera of Crangonidae can, I 
think, be traced back to Pontophilus, from which there have been 
two main lines of descent, originating respectively in Group I and 
in Group V. . 
In Group I of Pontophilus, as iias already been noted, the 
second leg is shorter than in the other groups and it is not difficult 
to understand how Sabinea, Owen, and Paracrangon, Dana, have 
arisen from it by successive steps. In Sabinea the pleopods are 
as well developed as in any species of Group I and possess a con- 
spicuous appendix interna; the second leg has, however, under- 
gone further reduction ; it is smaller than in any species of Pozito- 
philus and terminates simply, the chela being altogether sup- 
pressed. Paracrangon is apparently a further development on the 
same line. The second legs are entirely absent and the endopod 
in the last four pairs of pleopods, though large and well-formed, is 
without appendix interna. 
The other line of development has apparently arisen through 
forms similar to those of Group V and terminates in four branches, 
representing respectively the genera Crangon, 1,., Sclerocrangon, 
Sars, Argis, Kroyer (= Nectocrangon, Brandt) and Prionocrangon, 
Wood-Mason. In all these genera the endopod of the last four 
pleopods is greatly reduced, possesses a well marked basal segment, 
and is devoid of appendix interna. 
The evidence afforded by the development of the second leg 
is, in this case, rather difficult to interpret. In all the four genera 
named above it has a proportionately greater length than in any 
species of Pontophilus; in Prionocrangon, evidently a very highly 
specialised form, it terminates simply, while in the other three it 
is chelate. But it must be presumed that all Crangonidae have 

1 Ortmann, Zool. Fahrb., Syst., V, p. 530 (1890). 
