Dr. H. A. Nicholson on the Genus Climacograpsus. 381 
regarded as genuine varieties or as being dependent upon age 
or development. 
Fig. 5. 
TE t | 
BE 
d [4 
b a 
a. Base of C. bicornis, after Hall, showing a basal disk or bulb ; ma 
two diameters. b. Base of the same, showing an imperfect "basal c ud 
centic disk ; enlarged, from a specimen from the Llandeilo Shales of 
Glenkiln Burn, Dumfriesshire. c. Base of the ordinary form of C. bi- 
cornis in mfriesshire, without a central radicle ; enlarged. d. Base 
of the same, after Hall, showing the lateral spines flanking a central 
radicle (= Diplograpsus tricornis, Carr de 
In the first of these (fig. 5c), which may be regarded as the 
typical form of the species, the base is simply provided with 
wo curved spines or mucronate processes, which diverge from 
the lateral angles of the base and enclose an angle of from 40° 
to about 140°. As a rule, these basal processes form strong 
arcuate cornua ; but they are not unfrequently of the nature of 
slender spines, and are then usually straight or only slightly 
ae In any case they seldom attain a length of more than 
a line, 
In the second form (= Diplograpsus tricornis, Carr. ?), i 
addition to the two lateral spines, there is a a median mei US or 
“ radicle" (fig. 5 d), which varies in length from a mere tu- 
bercle to half a line or more. 
In the third form (fig. 55) there is developed round the 
lateral processes a corneous disk or bulb, of a crescentic shape, 
each horn of the crescent sometimes attaining, in large speci- 
sse a length of nearly three lines. This rare and singular 
ariety occurs not very uncommonly in a band of highly 
katit shale in Glenkiln Burn sshire 
In the fourth variety (fig. 5 a), which was Abit P 1 
Hall, the basal disk is very much more extensively develope 
osing its crescentic form, and extending both completely be- 
tween the basal spines and also up to the level of the third or 
fourth cell-aperture on each side. 
The frond itself of C. bicornis differs in no important parti- 
