Chap. IX.] 
LOWER SILURIAN GASTEROPODS. 
197 
such as Trochus, Turbo, Natica (or Pileopsis), and Patella. Of the latter genus 
an Irish species (P. Saturni, Goldfuss ?) is here figured, Foss. 40. f. 9. 
The families which appear to have existed during the Lower Silurian time 
were — Littorinidse, Pyramidellidse (in abundance), with Haliotidee, and a rare 
Patella. 
In the Upper Silurian are true Turbinidse (Euomphalus), with Calyptrseidae 
(Pileopsis and some allied genera). The Naticidae seem represented by one 
species, N. parva, Sow. ; they are not common until the Devonian period. 
Among the old Gasteropoda, the genus Euomphalus occurs, four or five species 
being known in the inferior strata (one is figured in PI. VII.). We find that 
Euomphalus, like many other genera, pervades the whole Silurian system ; but 
it is far less characteristic of the Lower than of the Upper Silurian. It is 
not certain that any one species extends upwards from the Llandeilo Flags into 
Wenlock strata. 
One of the most characteristic univalves of this age is the Murchisonia, more 
complete forms of which are given, in the annexed woodcut (Foss. 40. f. 6, 7) 
Fossils (40). Lower Silurian Gasteropoda. 
1. Maclurea Logani (?)' 
Salter ; from the'Lower Si- 
lurian of Ayrshire. Oper- 
culum of M. Logani, from 
Canada, f. 1 a. 2. Raphisto- 
6 ma Eequalis, Salter. 3. Mur- 
| chisonia obscura, Portl. 4. 
Ik Cyclonema rupestris, Eich. 
jjjr 5. Holopea concinna, M'C. 
6. Murchisonia gyrogonia, 
M'Coy. 7. M. subrotun- 
data, Portl. 8. Helmin- 
thochiton Griffithii, Salter. 
9. Patella Saturni, Gold- 
fuss? 
than any figured in my larger work, where they are called Pleurotomariae. In 
Portlock's ' Report ' several species of this genus are described from the Lower 
Silurian rocks of Tyrone: one of these, M. turrita, Portl., is very elegantly 
sculptured ; another, M. subrotundata, Foss. 40. f. 7, is conspicuous for its broad 
rough band. M. gyrogonia, f. 6, and M. simplex, M'Coy, are common in Wales, 
and there are many other similar species. Turritella is absent; the Lower 
Silurian shells resembling it are either smooth forms of Murchisonia, such as 
M. obscura, Foss. 40. f. 3, in which case they show the characteristic band, or 
they belong to the genus Holopella, in which the striae of growth run straight 
across the whorls, and are not bent into a notched or angular form. Several of 
the latter genus have been described from the Llandovery rocks, but they are 
comparatively rare in the Caradoc or the Llandeilo beds. They have in general 
an elongated and beaded shape, resembling the above-quoted Murchisonia ob- 
scura, Portl. ' Trochus lenticularis ' of the 'Sil. Syst.' pi. x. f. 2, and several 
similar discoid and angular univalves, are now referred to the genus Raphistoma, 
Hall. This genus is exceedingly common in the Lower Silurianrocks of America 
it also occurs in Scandinavia and Russia. R. aequalis, Salter *, found at Llandeilo, 
is here given, Foss. 40. f. 2 ; and others are known in the Silurian rocks of the 
south of Scotland and in Tyrone. 
* Mem. Geol. Surv. vol. iii. p. 271. 
