Chap. VEX] LOWER SILURIAN, N.W. HIGHLANDS. 
165 
a doubt to be a true Lower- Silurian Maclurea. The upper surface of the 
shell is shown in Foss. 27. f. 1, and its lower surface in f. 2. The species 
differs from M. magna (Hall) of North America in the slight convexity of the 
whorls beneath, and from M. Logani (Salter) of Ayrshire, Foss. 40. f. 1, by 
the greater number of turns of the spire. The operculum, Foss. 27. f. 3, is of 
a most peculiar form, as may be seen by comparing it with that of M. Logani, 
Foss. 40. f. la. It is almost spiral ; and this greatly helps to strengthen 
the opinion of the late Dr. S. P. Woodward*, that the genus may be one 
of the most ancient forms of the Rudista, a group uot previously known 
to occur below the horizon of the Oolitic formations. See also page 198. 
With this Maclurea, so decided in its bearing upon the age of the rocks 
in which it is imbedded, are associated other forms no less confirmatory of 
the identity of this group with the Lower- Silurian rocks of North America. 
They are here grouped in the same woodcut, in order to explain at a glance 
the general fossil contents of these Highland rocks. 
Ophileta compacta, Foss. 27. f. 4, the smooth Orthoceras with large com- 
pressed siphuncle, f. 6, others with an annulated surface, and a species of 
Oncoceras, f. 5, all closely resemble fossils of the Lower-Silurian rocks of 
North America, which range from the Calciferous Sand-rock up to the 
Fossils (27). Lower Silurian Shells from the North-western Highlands. 
1. Maclurea Peachii, Salter, upper side. 2. The same, lower surface. 3. The long, 
twisted operculum ; two views. 4. Ophileta compacta, Salter. 5. Oncoceras, sp. 
6. Orthoceras, with compressed siphuncle, like a Canadian species. 
Trenton Limestone, both inclusive. Thus, for example, although the Onco- 
ceras, f. 5, is specifically distinct from any published American species f, 
yet, when taken with the forms above enumerated, and particularly with 
Ophileta compacta, which is identical with a Canadian species, there can 
be no doubt that these Highland strata occupy the horizon assigned to 
them. It is also to be observed that these Scottish and American rocks 
resemble each other, to a great extent, in mineral composition as well as 
in their fossil contents. 
The lower quartz -rock of this group in Sutherland is characterized by 
* Manual of the Mollusca, p. 202. f See Hall, Pal. New York, vol. i. pi. 41. 
