42 CANADIAN FOSSILS. 



divergent at a much less angle. The hinge-line of Orthis is variable 

 in width, often considerably less than that of the shell. In Stro- 

 phomcna it is always as wide as the widest part of it. Orthis has 

 generally one, sometimes both valves convex. Strophomena has 

 both valves flattened, and generally one bent over the other. And 

 while the impressions of the blood-vessels in the mantle of Orthis are 

 radiating and but little curved upwards on the sides, those of 

 Strophomena take a wide upward bend, and quite encircle the ovarian 

 spaces. 



The range of the three genera, thus defined, is somewhat different. 

 Orthis is the earliest, beginning in the period of the Lingula flags, 

 and reaching upwards by one or other of its subgenera to the 

 Permian rocks. Leptcena began not quite so early, in the Llandeilo 

 flags, but maintained its position till the Oolitic period ; while 

 Strophomena, more restricted than either, commenced apparently at 

 the same period with Leptcena^ but is not known later than the 

 Devonian epoch. Some authors of repute (M'Coy, Woodward, etc.,) 

 regard Strophomena and Leptcena as mere subgenera, but I prefer, 

 as Mr. Thomas Davidson has done, to give them each generic rank. 



The separation of the four subgenera, — Orthis (Dalman), Orthisina 

 (D'Orbigny), Streptorhynchus and Platystrophia of King, is quite 

 convenient, and of geological value. The range of the true Orthis 

 is the most extensive, and the possession of an open triangular 

 foramen in one or both valves, distinguishes it from Orthisina, a 

 Lower Silurian form. In this both valves have a closed deltidium, 

 a small circular hole only being left in the apex of the larger valve. 

 Platystrophia, which has the form of a Spirifer, and an open foramen 

 in both valves, is Silurian only, while StreptorhyncMis, (which, in the 

 form of the teeth and hinge-plates) is most like Orthisina, has a wide 

 area, and is attached by the twisted beak. It is an irregular form, 

 characteristic of the Carboniferous and Permian strata. 



Locality. — Pauquette's Rapids, in tolerable plenty, with Stropho- 

 mena planumbona and S. filitexta (S. alter nata, seems to be absent). 

 Rhynchonella increbescens and Pentamerus (alrypa) hemiplicatits of Hall 

 are here. They are both Trenton fossils, and there are one or two 

 other species of Rhynchonella and of Orthis, of smaller size. 



