200 
SILUEIA. 
[Chap. IX. 
It must here be specially noted that in the progress of research no less than 
eight species of Orthocerata have, in England, Wales, and Ireland, been found to 
range from strata of the age of the Caradoc rocks to the Upper Silurian * , 
several of them even into the Upper Ludlow rock : e. g. 0. angulatum, Wahl. 
(O. virgatum, Sil. Syst.) ; 0. ibex, Sil. Syst. ; 0. subannulatum, Miinst. ; 0. 
Fossils (42). Lower Silurian Cephalopoda. 
1. Orthoceras vagans, 
Salter. 2. O. bilineatum, 
Hall. 3. O. tenuicinctum, 
Portl. 4. O. Brongniarti, 
Troost. 
Some of the forms of 
Orthoceras that are most 
common in the Lower Si- 
lurian rocks. 
annulatum, Sil. Syst. (0. undulatum of foreign authors) ; O. filosum, Sil. 
Syst. ; O. subundulatum, Portl. (Creseis Sedgwicki, Forbes) ; O. tenuicinctum, 
Portl. ; and O. primsevum (Creseis primseva, Forbes). There are probably 
others which have as great a range f. 
The singular Orthoceras bisiphonatum of the Sil. Syst., only found in one 
locality, near Llandovery in South Wales (PI. XI. f. 5), appears to have two 
siphuncles, and belongs, in the opinion of Mr. Salter, to a distinct genus, for 
which he has proposed the name of Tretoceras J. It will be again noticed 
under the Llandovery formation. Oncoceras has been already mentioned, 
p. 165. 
There are several species of the genus Lituites in these old strata. L. cornu- 
arietis, Foss. 43. f. 2, and L. Hibernicus, f. 3, are good examples of our British 
species. The former is also found in Scandinavia. L. anguiformis, Salter, is 
a third ; and there are other species in the Llandovery rocks. Cyrtoceras, 
Fossils (43). Lower Silurian Cephalopoda. 
i 
3 / 
1. Cyrtoceras ingequisep- 
tum, Portl. 2. Lituites 
cornu-arietis, Sow. 3. L. 
(Trocholites) Hibernicus, 
Salter. 
Curved forms of Cepha» 
lopoda, rare in British 
Lower Silurian strata. 
which is distinguished by being much less curved than Lituites, includes several 
species in Britain : one, C. inasquiseptum, Portl., from Ireland, is here repre- 
sented, f. 1 ; and some others are figured by Portlock ; and one of the Ayrshire 
Cephalopods is probably the Cyrtoceras multicameratum, Hall, of the United 
States. 
* Synopsis of the Palaeozoic Fossils, by Sedg 
t See Plates XXVI. to XXIX., and Chap. . 
I Quart. Journ. G-eol. Soe. vol. xiv. p. 179. 
aek and M'Coy, pp. 313 &c. 
. for some of these species. 
