Chap. IX.] LOWER SILURIAN CEPHALOPODS ETC. 
199 
f. 5, may probably be compared with the modern genus Atlanta — if its thin shell, 
triangular section, and finely striated surface do not betray affinity with the 
group next mentioned. 
There are certain genera, now clearly ascertained to be forms of Pteropoda — 
one of the inferior groups of Mollusca. Of Conularia, for example, a most beau- 
tiful ornate form, there are two or three species in the Lower Silurian. C. elon- 
gata, figured in the next woodcut, Foss. 41. f. 3, is frequent in Ireland ; and C. 
Sowerbyi, PI. XXV. f. 10, ranges upwards from the Caradoc strata to the Ludlow 
rocks. M. Barrande has figured many curious and large forms of Conularia from 
the Silurian rocks of Bohemia, one of which is spirally curved. Theca, a genus 
exceedingly like the modern Clio, has two species at least in the lower division, 
viz. T. triangularis (Orthoceras of Portlock), Foss. 41. f. 2, and another species, f. 1 
(T. triangularis of Hall), to which Mr. Salter applies the name T. reversa. It is 
found in North America, Scotland, and Wales, and is distinguished by hav- 
ing the dorsal (and not the ventral) surface convex j otherwise it is much like T. 
Forbesii, Sharpe. 
Fossils (41). Heteropod and Pteropod Mollusca of the Lower Silurian Strata. 
H transversa, Portlock. 
m 5. Ecculiompbalus 
I Bucklandi, Portl. 6. 
I Bellerophon pertur- 
§ batus, Sow. 7. B. 
I acutus. 8. B. dila- 
l tatus, Sow. 
Figs. 5 and 8 are species often 3 inches wide. The figures are much reduced. 
Pterotheca is a genus proposed (Report Brit. Assoc. 1852) for a wide shell 
like Cleodora : Pt. transversa is here given, f. 4. Pt. undulata, Salter, is a 
large and beautiful fossil found in North Wales and Shropshire. 
It is to be remarked that these ancient forms of their order were gigantic in 
comparison with their modern representatives. 
Cephalopodous or chambered Shells of the genera Orthoceras, Cyrtoceras, and 
Lituites occur in the Lower Silurian rocks. In the original Silurian region 
they are, indeed, less abundant in the lower than in the upper divisions of the 
system ; but on the continent of Europe, particularly in Scandinavia, where 
they occur under different conditions, and in limestone, they are more frequent 
in the Lower than the Upper Silurian. Among the earlier developed 
British species are to be noted Orthoceras politum, M'Coy, a large smooth shell 
seen by myself in situ in Ayrshire * ; 0. vagans, Foss. 42. f. 1 j O. bilineatum, 
f . 2 ; O. tenuicinctum, f. 3 : the latter is a beautiful fossil. Another species 
characteristic of the Lower Silurian strata, and having a large lateral siphuncle, 
is 0. Brongniarti of Portlock, f . 4 ; it is supposed to be identical with one from 
North America, where similar forms are common in the Lower Silurian rocks. 
The great Orthoceras of the same strata of Scandinavia and Russia, 0. vagi- 
natum, Schlotheim, is occasionally three feet long ; it has been found in Scot- 
land f. Smooth Orthocerata, on the contrary, with slender central siphuncles, 
are not confined to either the upper or lower division. 
1. Theca reversa, 
Salter (dorsal view). 
2. T. triangularis, 
Portl. (dorsal and 
ventral sides). 3. 
Conularia elongata, 
Portl. 4. Pterotheca 
* See Quart. Journ. Geol. Soc. vol. vii. pi. 10. 
t lb. p. 177. 
