THE LOWER SILURIAN PERIOD. 



Ill 



Modiolopsis, Ctenodonta, Orthonota, Palcearca, 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 Heteropods are also very 

 abundant, the principal forms belonging to Bel- 

 lerophon and Madurca. 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 Madurea (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- 

 lum." The Lower Silurian Pteropods, or "Wing- 

 ed Snails," are numerous, and belong principally 

 to the genera Theca, Conularia, and Tentaculites, 

 the last-mentioned of these often being extremely abundant in 

 certain strata. 



Lastly, the Lower Silurian Rocks have yielded a vast number 



Fig. 52. Mur- 

 cliisonia gracilis, 

 Trenton Lime- 

 stone, America. 

 (After Billings.) 



it views of Rellero/>hon 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 Cephalopods 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 



