ACTINOCERATID^. 
167 
centre of the shell, so as to give in section a somewhat hilohed 
form.” (Salte7\) 
Remai'Tcs. It is impossible to say anything decisive about fossils 
in the condition in which these are. The specimens representing 
this species in the JS’ational Collection are most of them weathered 
loiigitudinal sections, much injured by mineral agency, their walls 
being replaced by orbicular chalcedony (“ Beekite ”), so that all the 
original structure has been obliterated. Prof. Blake (Joe. cit. p. 82) 
describes the siphuncle in this species as having a comparatively 
large-sized inner tube, untouched by the septa, and slightly folded 
longitudinally, and, he adds, “ these features are seen in all 
specimens showing the interior, and proving that we have here a 
very remarkable type with a siphuncle of two distinctly preserved 
layers, the outer one being separated by obstructing deposits from 
the inner one.” 
Of this “ inner tube ” (endosiphon ?) there are pretty clear indica- 
tions in the specimens before me, and the “ shallow groove ’’noticed 
by Salter was doubtless the remains of a similar organ split longi- 
tudinally, in which condition it is generally found. 
Horizon. Durness Limestone ^ ( =Calciferous(?) of I^’orthAmerica). 
Localitij. Durness, Sutherlandshire. 
Fairly well represented in the Collection. 
' J. E. Marr (‘ Classification of the Cambrian and Silurian Eocks,’ p. G7, 1882) 
states that the Durness Limestone “ seems to be intermediate in character 
between the Orthoceras-Limestone of Sweden and the Craighead Limestone of 
Girvan. A ridge of pre-Cambrian rocks extends through Central Scotland, and 
the lowest Girvan beds were deposited against it on the south, and the Durness 
Limestone on the north. The Durness Limestone is probably therefore homo- 
taxial with the Orthoceras-Limestone of Sweden, but of somewhat later date,” 
This places tlie former upon a higher horizon than that to which Mr. Peach 
assigns it. In a Presidential address delivered before the Eoyal Physical Society 
of Edinburgh (1885) the latter makes the following observations regarding the 
age of the Ordovician Strata of the Eorth-west Highlands of Scotland : — “ . . . 
the fossils are of an American type, and do not resemble those found in the 
contemporaneous deposits of Wales and England. Indeed, so far as the order 
of succession of the beds and their fossil contents are concerned, we have an 
almost exact counterpart of the strata exposed along the axis of older Palseo- 
zoic rocks, stretching from Canada through the eastern States of North America. 
In the latter region, the Silurian strata of Sutherlandshire are represented by 
(1) the Potsdam Sandstone, always described as being vertically piped by 
ScoUthus, like the ‘ pipe-rock (2) the Calciferous group ; and (3) part of the 
Trenton Limestone. Now, it might be contended that this remarkable identity 
in the order of succession of the beds and fossil contents in these two widely 
separated areas points to contemporaneous deposition; and, on the other hand, 
it might be argued with greater probability that the beds are homotaxial, or in 
