372 Dr. H. A. Nicholson on the Genus Climacograpsus. 
arrangement of parts is shown in certain of the Diplograpst 
(e. g. in D. putillus, Hall, and D. tamariscus, Nich.). 
the whole, then, Climacograpsus differs from Diplograpsus 
merely in the fact that the cell-apertures are not placed at the 
extremity of projecting denticles. This difference is not, per- 
haps, one of great structural importance ; but it communicates 
such a peculiar aspect to the whole group of the Climacograpst 
that for the present the distinctness of the genus may well be 
maintained. 
Taking Climacograpsus teretiusculus, His., as the type, the 
following may be described as the structure of the genus, 
though it is probable that some of the species differ from this 
in respects more or less important :— 
The frond (fig. 1) is diprionidian, having cellules on each 
side, and is cylindrical in shape, tapering more or less rapidly 
towards the base. It is composed of two simple unicellular 
stipes placed back to back, their internal or dorsal walls 
coalescing to form a single vertical septum (fig. 1 g), along 
the centre of which runs a delicate solid axis, in the form of a 
fron 
the pom corresponds to one of the stipes composing the. 
in ? 
ing together the two halves of the pct may be taken to 
grapsus teretiusculus is conclusively proved by an extensive 
suite of specimens, preserved in relief, which 1 have obtained 
from the Mudstones of the Coniston series of the north of 
