40 Fossils of the Lower Sihirian Rods of Canada* 



numbers of living things ; fossil plants, corals, echinoderms, mollusks and 

 trilobites are to be found in greater or less abundance wherever these 

 remarkable rocks are exposed upon the surface. The whole country 

 between the rivers Ottawa and St. Lawrence, comprisingthe greater portion 

 of the Counties of Carleton, Kussell, Prescott, Glengarry, Dundas, Stor- 

 mont, Leeds, Grenville, and also small areas in Lanark and Eenfrew, are 

 overlaid by enormous sheets of those limestones from 200 to 600 feet in 

 thickness, crowded full of organic remains. There are vastly more 

 animals buried in one cubic mile of the Trenton limestone than there are 

 living at any one time upon the whole continent of America. They are all 

 of extinct species, — nearly all of extinct genera, and many of them, such as 

 the cystideans, orthoceratites, and trilobites, of orders which became 

 wholly exterminated, myriads of ages since. 



In the following article we shall give figures and descriptions of some 

 • of the most abundant and easily recognized species. 



Fig 1. Orihis testudinaria. 



Fig 2. Leptena sericia. 



Orthis testudinaria. — Fig 1 represents a common species of Orthis, s 

 genus which consists of small fossil bivalve shells, generally of a circular shape, 

 and with one valve more flattened than the other. In this species, the ven- 

 tral valve above figured is the most convex or rounded of the two. At the 

 upper side or upon the hinge line it projects into a small sharp or moderately 

 obtuse beak. The dorsal valve is nearly straight along the hinge line, flat- 

 tened or but slightly convex, and in most specimens with a shallow depres- 

 sion which extends from the centre above to the base. The smfaces of both 

 valves are covered with fine elevated lines or ridges Avhich radiate from the 

 beak downwards and outwards. Towards the margin these lines bifurcate, 

 and in very perfect specimens are crossed by numerous delicate concentric 

 thread-like strise. Often the circular margin at the base is thickened, and 

 appears as if several shells were laid one within the other. 



This little fossil is usually of the size of Fig 1, or somewhat less, and 

 the specimens are most frequently found with the valves united and closed 

 in their natural position. It is the most abundant of all the species of this 

 genus found in the Lower Silurian rocks. It is generally seen partly im- 

 bedded in the surfaces of the strata of limestone, but often when it occurs in 

 the shale between the beds of the rock it can be obtained perfectly separated 

 and in great numbers. It has a very wide geographical range, as it is found 

 abundantly in the Lower Silurian rocks of Europe, as well as in those of 

 America. Professor Hall says, " a comparison of a Swedish specimen of 



