Distribution of the Rhabdophora. 255 
Professor Hall * confined his observations to a note of the 
vertical range of the Rhabdophora in American deposits. 
Their earliest appearance he showed to be in the Quebec 
Group of Lower Canada, at or near the general horizon of 
the Calciferous Sandstone of New York; they there at¬ 
tained their maximum development, both in genera and 
species. lie distinguished also a second but less prolific 
horizon, viz. that of the Utica Shales and the Slates of Nor¬ 
man’s Kiln in the valley of the Hudson river. These he 
placed at the summit of the Ordovician. The only Silurian 
rocks known to Hall as affording Rhabdophora were the 
shales of the Clinton formation, at the base of the Niagara 
group. 
Mr. Salter was probably responsible for the lists of fossils 
appended to the several editions of Murchison’s 1 Siluria.’ How 
naturally he felt impelled to assign every prolific graptolite- 
bearing stratum to the Llaudeilo has been already pointed 
out. Even as late as 1868 the influence of this feeling was as 
apparent as ever. A glance at the list of Graptolites in the 
Table of British Silurian fossils, in the fourth edition of 
1 Siluria,’ will make it evident that, if we exclude the few 
admitted Wenlock forms and the half-dozen species from the 
beds of Pomeroy and Girvan (whose occurrence in strata 
crowded with Bala Crustacea made it imperative upon the 
conscientious palajontologist to give them a place in the Cara- 
doc column), almost all the British Rhabdophora are assigned 
to the Llandeilo. Of the fifty-one species there cited, forty 
are placed in the Llandeilo column; and thirty-eight of these 
were tabled as peculiar to that formation. Three species 
(Climacograptus scalar is : Iiis., Monograptus convolutus , His., 
and Monograptus Sedgivickii , Portlk.) are given as common 
to the Llandeilo and Caradoc beds. Climacograptus bullatus, 
Salt., Monograptus Conybeari, Portlk., Monograptus gries- 
tonensis , Nicol, are noted as peculiar to the Caradoc, in all 
probability for the reasons given above. Beyond Mono¬ 
graptus priodon , Bronn, whose range is mentioned as ex¬ 
tending from the Caradoc to the Ludlow, no Graptolite is 
noted as common to both the Ordovician and Silurian. To 
the latter only two other species are assigned— Monograp>tus 
Flemingii , Salter, and Retiolites Geinitzianns, Barr. 
Professor H. A. Nicholson has made this question the sub¬ 
ject of several important memoirs. In a paper contributed to 
the 1 Annals and Magazine of Natural History’ in 1868, he 
treated of the “Distribution in Time of the British Genera and 
* Ctrapt. Quebec Group, pp. 51-oS. 
