123 



specimen has a rounded fold in the front margin which becomes obsolete 

 at one third the length of the shell. 



This species by its projecting ears, narrow areas, and striated hinge 

 teeth is most closely related to S. Leda, from which it differs in being 

 four times the size. It has so much of the aspect of S. alternata, that at 

 present we have no means of distinguishing it from that species without an 

 examination of the hinge area and teeth. 



Locality and Formation. Middle Silurian, Anticosti, associated with 

 Pentamerus oblongus. 



Collector. J. Richardson. 



The above four species, S. nitens, S. Ceres, S. Leda, and S. Philomela 

 are closely allied to S. alternata. The three following have the ventral 

 valve concave, and belong to a very different group, of which jS. filitexta 

 (Hall) may be regarded as a typical form. 



STEOPHOMENA FLUCTUOSA. N. s. 



STROPHOMENA FLUCTUOSA. Can. Nat. and Geol., Vol. V., p. 57, Feb. 1860. 



Fig. 102. 



Fig. 102. Strophomenajluctuosa. a, section, the dotted line represents the plane of the 

 lateral margin, and it is drawn to shew that the area of the concave or ventral 

 valve is at a right angle to it. 



Description. Triangular, or semi-oval, usually widest at the hinge-line, 

 and more or less narrowly rounded, pointed, trilobed, or nasute in front. 



Dorsal valve convex, the visceral disc being in general equal to one 

 third the superficies of the whole valve, nearly flat, the remainder abruptly 

 curved down all round so that the lower half of the length of the shell is 

 sometimes at right angles with the upper half. The cardinal angles more 

 or less compressed and often a little reflected, usually forming angular or 



