54 



obscure radiating striae, ten or twelve in the width of one line, crossed by 

 fine concentric striae about the same distance apart. This arrangement of 

 the striae gives to the surface a finely punctuated aspect. 



Length, 6 or 7 lines ; width, 5 or 6 lines. 



The shell is thin and of a light brown or dun colour. When a little 

 worn, it has a smooth, glistening appearance, and the punctures and striae 

 are then scarcely visible. 



Trematis cancellata (G. B. Sowerby,) must be closely allied to this 

 species. It is thus described : " Shell orbicular, very flat, being more 

 gibbous near the posterior extremity ; surface covered with close-set, ele- 

 vated lines radiating from the apex, which are crossed by the elevated 

 lines of growth, so that the entire surface has a finely reticulated appear- 

 ance ; the fissure in the ventral valve is small and close to the hinge ; 

 shell very thin ; length and breadth, f of an inch." (Sharpe, Jour. Geo. 

 Soc., Vol. 4, p. 69.) I have not seen Sowerby's original description and 

 figure, but from the above it would appear that T. cancellata has a small 

 foramen, while T. Huronensis has a large one, and also the radiating and 

 concentric striae not elevated. 



Locality and Formation. Pallideau Islands, Lake Huron. Black 

 River limestone. 



Collector. A. Murray. 



Genus ARTHKOCLEMA. (N. gen.) 



Cf-eneric characters. The only species of this genus at present known 

 consists of a cylindrical jointed stem with several long slender branches 

 which are also usually jointed. The surface exhibits numerous small oval 

 pores resembling those of Ptilodictya. 



These fossils have somewhat the appearance of species of Ptilodictya. 

 but differ in having the stems cylindrical instead of compressed. 



Generic name from " arthron " a joint and " klema " a twig. 



ARTHROCLEMA PULCHELLA. (N. sp.) 



Fig. 60. 



Description. The central or principal stem of this species is from 3 to 

 6 inches in length and from J a line to 2 lines in thickness. The joints 

 are distant from 2 to 4 lines from each other. There are two branches to 

 each joint situated exactly opposite to each other on the opposite sides of 

 the mam stem. These also send off smaller branches from their opposite 

 sides, the whole being arranged in one plane like the mid-rib and veins of a 

 leaf. The young branches are not jointed but the full grown ones are. 



