198 
SILUEIA. 
[Chap. IX. 
Holopea, Hall, though rather an obscure genus of smooth rounded uni- 
valves, includes many Lower Silurian species. One is figured in PI. VII. f. 4, as 
H. striatella (Littorina, Sil. Syst.) ', another, H. concinna, is represented here, 
Foss.40. f.5. 
Other shells formerly called Turbo, e. g. T. rupestris, Eichw., Foss. 40. f. 4, 
and T. sulcifer, Eichw., T. crebristria, M'Coy, &c, belong to an extinct genus 
(Cyclonema of Hall) allied to Holopea. Cyclonema rupestris occurs in the Lower 
Silurian limestones of Ireland, and often still retains the coloured bands that de- 
corated the shell when alive *. This feature will be dwelt upon in the concluding 
Chapter, as possibly limiting the depth of water at which such shells lived. 
Though not yet detected in England or Wales, Maclurea (Hall), a character- 
istic genus of the Lower Silurian rocks of North America, has been found in 
strata of the same age in Scotland. The shell (Foss. 40. f. 1) appears to be 
sinistral or reversed, the top flattish, and the base umbilicate. In all probability, 
however, it should be viewed the other way upwards, and so appear with a 
sunken or concealed spire. Its mouth is closed by a remarkable operculum, of 
which f. 1 a shows the inner side. I collected a species of this genus in the 
Lower Silurian strata of Ayrshire, associated with Orthides, Trilobites of the 
genus Asaphus, and other Lower Silurian fossils. A Canadian species, approach- 
ing to M. magna of the United States of North America, and named by Mr. 
Salter, after the able geologist who discovered it in Canada, Maclurea Logani, 
is the form to which our British fossil is most allied. The operculum figured 
(Foss. 40. f. 1 a) is from that species. Another species, M. Peachii, Salter, has 
already been noticed in describing the fossil contents of the Lower Silurian rocks 
of the Highlands of Scotland (p. 165). 
Bellerophon, a palaeozoic form of the Nucleobranch Mollusks classed by some 
naturalists with Heteropoda, is one of those genera which specially link to- 
gether the Lower and Upper Silurian divisions in one system of life. Thus B. 
carinatus, PI. XXXIV. f. 8, and B. dilatatus, Foss. 41. f. 8, are common to both 
divisions, ranging from the Caradoc Sandstone to the Ludlow rocks in Ireland 
and Wales. On the other hand, there are several species which as yet are 
known only in the lower division, and are very characteristic of it. B. acutus 
(PI. VII. f. 8), B. bilobatus (figured at page 68, Foss. 13. f. 10), and B. nodosus 
(f. 11) are among the common fossils of the lower part of the Caradoc Sand- 
stone. One of these species, B. bilobatus, Sow., is equally characteristic of 
the same deposits in North America, Spain, and Bohemia. B. perturbatus, 
Foss. 41. f. 6 (Euomphalus, Sil. Syst.), is one of the common fossils of the black 
Llandeilo slates of Wales. There are other less-known species, and some are 
yet unpublished. 
Certain naturalists regard the genus Bellerophon as the shell of a Cephalo- 
podous animal, differing from the ordinary forms of that class in the want of 
septa or partitions within the shell. Although there is some ground for the 
supposition, these shells are now usually believed to be Nucleobranchiata (or 
Heteropoda), allied to the floating Carinaria or Glass-shell, which they much 
resemble both in form and sculpture. 
The large Irish Lower Silurian fossil, Ecculiomphalus Bucklandi, Foss. 41. 
* Cyclonema rupestris, Eichw., from the Lower thorns, we may by this fact strengthen the infer- 
Silurian limestone of the Chair of Kildare, Ire- ences arrived at in Chapter XX, from other 
land, has concentric bands of colour, and an Ortho- data, that some of the Silurian sea-beds, extending 
ceraa of the Caradoc Sandstone exhibits very di- over considerable areas, were formed in compa- 
stinct longitudinal stripes. Wow as Edward Forbes ratively shallow water. Mr. Gwyn Jeffreys, how- 
(Proceed. Eoy. Soc. vol. vii. p. 21) has shown that ever, finds that coloured shells exist at greater 
coloured shells, striped or banded, do not usually depths ; so that the conclusion must be adopted 
degeegil to greater depths than from 15 to 20 fa- with caution. 
