174 Silurian Fossils of Canada, 



towards tlie apex wliile the ventral tliird slopes so as to form an 

 obtusely rounded right angle with the dorsal two thirds. The 

 aperture is thus obscurely trilobed, the ventral lobe being the smal- 

 lest and forming a deep narrow sinus in the ventral margin. At 

 the aperture the greatest transverse width is six lines in the spe- 

 cimen on which the species is founded ; the dorso-ventral diameter 

 eight lines. At seven lines (from the most prominent point of 

 the aperture on the side) we find the greatest transverse diameter 

 which is here seven lines and a half and the dorso-ventral diame- 

 ter nine lines. At the length of one inch and a fourth the 

 dorso-ventral diameter is reduced to between five and six lines. 

 The remainder of the specimen is not preserved. The depth of 

 the chamber of habitation is ten lines. The first five septa occupy 

 six lines of the ventral margin, but at the middle of the lateral 

 aspect four lines and a half, Siphuncle about one line in diame- 

 ter and apparently in contact with the shell along the median line 

 of the ventral side. The surface of the shell is covered with fine 

 stride or rather small smooth continuous wrinkles which encircle 

 the tube following the curves of the aperture. These wrinkles 

 vary in size, but in general there are five or six in the width of 

 one line. 



The majority of the species of this genus have the siphuncle on 

 the inside or close to the shell on the side of the concave curva- 

 ture. M. Barrande, however, has one species P. perversum in 

 which it lies close to the outer curve,* The aperture is not so 

 strongly trilobed as it is in Upper Silurian and Devonian species. 



I believe this is the only Phragmoceras known in the Lower 

 Silurian Rocks. 



Locality and Formation, — The specimen figured was found on 

 Le Cloche Island, Lake Huron, in the Black Eiver Limestone. 

 Two other fragments have been collected, one at La Petite Chau- 

 diere Bapids and the other at Pauquettes Bapids, Ottawa Biver, 

 in the same formation 



Collectors. — Sir "VV. E. Logan, B. Bell, E, Billings. 



Orthoceras tener. N. s. 



Descrij^fion. — Small, section sub-oval ; broadly rounded on the 

 •dorsal and very gently convex on the ventral aspect ; sides nar- 



* See translation of Barrande's note on the Silurian Cephalopoda of 

 Bohemia, Jour. Geo. Soc. Vol. X. Translations p. 21. 



