196 
SILURIA. 
[Chap. IX. 
cies, Ctenodonta varicosa, Foss. 39. f. 4, and Ct. obliqua, f. 6, are frequent in the 
lower division, — the former in Wales, the latter in North Ireland. It is most 
probable that these are all referable to Nucula-like shells (Ctenodonta [Telli- 
nomya, Hall], Bep. Brit. Assoc. 1851, Trans. Sect. p. 63) which, like the Isoarca 
Fossils (39). Lamellibranchiata. 
1. Modiolopsis postlineata, 
M'Coy. 2. M. expansa, 
Portlock. 3. M. modiolaris, 
Conrad. 4. Ctenodonta va- 
ricosa, Salter. 5. Lyrodesma 
plana, Conrad. 6. Cteno- 
donta obliqua, Portlock. 7. 
Pleurorhynchus dipterus, 
Salter. 8. Ambonychia Tri- 
ton, Salter. 
of Minister, have the ligament external. Lyrodesma plana, Foss. 39. f. 5, which 
belongs to the same group of shells, is found in the limestone at Bala. 
Orthonota (Cypricardia of the Sil. Syst.) is a genus more frequently met with 
in Upper Silurian rocks ; but some of its species are Lower Silurian. 0. nasuta 
occurs in the Caradoc of Horderley, and is represented in the woodcut, Foss. 13. 
f. 12 (p. 68). Oardiola is rare, but not wholly absent. In the preceding illustra- 
tion (Foss. 39), a rare fossil is figured, Pleurorhynchus dipterus, f. 7, which, though 
unknown in England and Wales, has been found by Mr. J. Carrick Moore in the 
Lower Silurian rocks of Ayrshire, and by the Government Geologists at the 
Chair of Kildare, in Ireland. To these may be added a remarkable genus found 
in Montgomeryshire, the Megalomus of Hall. It is a thick and clumsy shell, 
something like the Megalodon of the Devonian rocks, but with less defined 
teeth. Hall's species is from the Clinton group (Llandovery rocks) ; the British 
one is from the true Lower Silurian at Horderley. 
The Gasteropods, or Univalves, are now declared by naturalists to belong 
mainly to genera which are extinct. Such is the Murchisonia of d'Archiac and 
de Verneuil, formerly included in the genus Pleurotomaria. Such also are Ho- 
lopea and Raphistoma of the American palaeontologists, Loxonema and Ma- 
crocheilus of Phillips, and Euomphalus ; and the few species of Turritella for- 
merly quoted belong to a different genus, the Holopella of M'Coy. 
It is important to remark that certain shells which abound in the younger 
Secondary and the Tertiary deposits, and at the present day, viz. true species of 
Nerita, Pleurotoma, and Buccinum, have not yet been found in these ancient 
rocks *. Some few recent genera of Gasteropods, however, may still be quoted, 
* The reader maybe surprised to find numerous 
species of the three last-mentioned genera spoken 
of in the original ' Silurian System ; ' but it is 
only of late years that the distinctions have been 
drawn by which such genera have been excluded 
from the Silurian rocks, and the accuracy of the 
exclusion is now agreed upon by most naturalists, 
although these shells have much the external ap- 
pearance of the above-mentioned genera. Euom- 
phalus is probably a very near ally of the modern 
genera Delphinula and Skenia: it occasionally 
preserves its operculum and even the nacreous 
lustre of its interior. These changes, made by 
palaeontologists in the generic names, have an 
important bearing on the philosophy of the sci- 
ence, as the determination of the correct affinities 
of these fossils may lead us to understand the con- 
ditions under which they lived, — it having been 
shown conclusively, by the late Ed. Forbes, that 
certain conditions of depth and of sea-bottom were 
essential to the presence of one group of animals, 
while another assemblage might be indifferent to 
them. See also Dr. Bigsby's Tables, Quart. J ourn. 
Geol. Soc. vol. xv. pp. 259 &c. 
