112 



HISTORICAL PALEONTOLOGY. 



which, in the living condition, passed a muscular cord attach- 

 ing the shell to some foreign object. The genus Strophomena 

 (fig. 50, d, and 51, a and b) is very like Orthis in general char- 

 acter ; but the shell is usually much flatter, one or other valve 

 often being concave, the hinge-line is longer, and the aperture 

 for the emission of the stalk of attachment is partially closed 

 by a calcareous plate. In Lept&na, again (fig. 51, e), the shell 

 is like Strophomena in many respects, but generally compara- 

 tively longer, often completely semicircular, and having one 

 valve convex and the other valve concave. Amongst other 

 genera of Brachiopods which are largely represented in the 

 Lower Silurian rocks may be mentioned Lingula, Crania, 

 Discina, Treinatis, Siphonotreta, Acrotreta, Rhynchonella, and 

 Athyris; but none of these can claim the importance to which 

 the three previously-mentioned groups are entitled. 



The remaining Lower Silurian groups of Mollusca can be 

 but briefly glanced at here. The Bivalves (Lamellibranchiata} 

 find numerous representatives, belonging to such genera as 

 Modiolopsis, Ctenodonta, Orthonota, Palaarca, 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 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- 

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

 ed Snails, " are numerous, and belong prin- 

 cipally to the genera Theca, Conularia, and 

 Tentaculites, the last-mentioned of these often being extremely 

 abundant in certain strata. 



Fig. 52. Mur- 

 chisoniagracilis, 

 Trenton Lime- 

 stone, America. 

 (After Billings.) 



