376 Dr. H. A. Nicholson on the Genus Climacograpsus. 
(as shown in fig. 1 a & 4) intermediate forms exist in great va- 
ie ing in every possible way with the exact portion of 
the surface of the frond which may be exposed to view. 
The entire genus Climacograpsus appears, in Britain at any 
rate, to be exclusively confined to the lower division of the 
Silurian rocks. In the Skiddaw Slates three of the five 
British species have been detected by myself, namely, C. an- 
tennarius, Hall, C. bicornis, Hall, and C. teretiusculus, His. 
In the Upper Llandeilo rocks we have C. teretiusculus, C. 
bicornis, C. tuberculatus, Nich., and C. innotatus, Nich., the 
two former being highly characteristic of the formation. In 
the Caradoc rocks only C. teretiusculus has hitherto been de- 
tected; but it is extremely abundant in some portions of this 
series. Above the Caradoc rocks the genus is represented 
solely by a single example of C. teretiusculus, discovered by 
Professor Harkness in the Lower Llandovery rocks of Haver- 
rdwest. 
Climacograpsus teretiusculus, His., sp. 
Prionotus teretiusculus, His. Leth. Suecica, Supp. 2, t. 38. fig. 4. 
Diplograpsus rectangularis, M‘Coy, Ann. Nat. Hist. ser. 2. vol. v. p. 271; 
Pal. Foss. p. 8, pl. 1 5. fig. 8. 
Pete teretiusculus, Salt. Quart. Journ. Geol. Soc. vol. viii. pl. 21. 
gs. . 
Climacograpsus teretiusculus, Nich. Quart. Journ. Geol. Soc. vol. xxiv. 
p. 139, pl. 5. figs. 11-18 ; ibid. pp. 528, 529. 
Climacograpsus scalaris, Carruthers, Geol, Mag. vol. v. p. 181. 
Climacograpsus minutus, Carruthers, ibid. p. 132. 
Having already described the appearances which are ordi- 
narily presented by this species, it will not be necessary for 
o more here than simply to notice the characters of the 
base, as itis by these that C. teretiusculus is distinguished 
from the closely allied C. bicornis. 
n normal specimens of C. teretiusculus the solid axis is 
prolonged proximally below the base of the frond for a greater 
or less distance in the form of a filamentous cylindrical radicle. 
This is the ordinary condition of the base; and the only va- 
riation in this respect which is at all common relates to the 
length of the proximal extension of the axis. In a great 
many individuals (fig. 2a) the radicle is extremely short, not 
exceeding from } to 1 line or 2 lines, and this though the 
frond may be from 4 to 1 inch in length or even more. In 
other individuals, again (fig. 1 a,b), the length of the radicle is 
much greater, varying from } inch to as much as 1} inch, the 
frond itself reaching sometimes a length of as much as 23 
inches. "These variations, however, are probably due merely 
to the age of any given individual. 
Several departures, however, from the above simple state of 
