THE | LOWER SILURIAN PERIOD. i 
Modiolopsis, Ctenodonta, Orthonota, Palearca, Lyrodesma, Am- 
bonychia, and Cleidophorus. ‘The Univalves (Gasteropoda) are 
also very numerous, the two most important genera being 
. Murchisonia (fig. 52) and Pleurotomaria. In both these groups 
the outer lip of the shell is notched ; but the shell 
in the former is elongated and turreted, whilst in 
the latter it is depressed. The curious oceanic 
Univalves known as the /eferopods are also very 
abundant, the principal forms belonging to Bed- 
lerophon and Maclurea. In the former (fig. 53) 
there is a symmetrical convoluted shell, like that 
of the Pearly Nautilus in shape, but without any 
internal partitions, and having the aperture of- 
ten expanded and notched behind. The species 
of Maclurea (fig. 54) are found both in North 
America and in Scotland, and are exclusively 
confined to the Lower Silurian period, so far 
as known. ‘They have the shell coiled into a 
flat spiral, the mouth being furnished with a 
very curious, thick, and solid lid or “opercu-  _ Fa: 52-—4u- 
lum.” The Lower Silurian Preropods, or “‘Wing- Treen tne 
ed Snails,” are numerous, and belong principally (ane: Bille 
to the genera Zheca, Conularia, and Tentaculites, 
the last-mentioned of these often being extremely abundant in 
certain strata. 
Lastly, the Lower Silyrian Rocks have yielded a vast number 
Fig. 53.—Different views of Bellerophon Argo, Trenton Limestone, Canada. 
(After Billings.) 
of chambered shells, referable to animals which belong to the 
same great division as the Cuttle-fishes (the Cephalopoda), and 
of which the Pearly Nautilus is the only living representative at 
the present day. In this group of Cephalofods the animal 
possesses a well-developed external shell, which is divided 
into chambers by shelly partitions (“septa”). The animal 
lives in the last-formed and largest chamber of the shell, to 
