NAYADES. 109 



terior side slightly gaping, much elongated, but not subrotund 

 and subangulated ; produced by the somewhat elevated ridge, 

 which diverges from the umbones to the extreme point of the 

 valve ; cardinal teeth small, compressed, minutely serrated ; 

 double, and lamelliform, elongated in the left valve, with a 

 deep, perpendicularly sulcated sinus in its side, for the recep- 

 tion of the tooth of the right valve ; the portion of the tooth 

 next the umbo somewhat acute ; lateral tooth long and double 

 in the left valve, single and sunk towards the umbo, and nearly 

 parallel ; lateral tooth of the right valve single, depressed 

 towards the umbo, but ascending, and more acute, as it 

 diverges from the umbo ; inside bluish-white, and iridescent ; 

 muscular impressions very deep; pallial impression well defined; 

 outside of a dark olive-green, and with rather deep, concentric 

 wrinkles. 



Figs. 3 and 4, pi. XXI, exhibit the external and internal 

 appearance of the teeth. 



This species differs from U. pictorum, in its general length 

 being more uniform, and in being broader at the posterior 

 side, and less acute ; in the teeth being much smaller, and less 

 elevated, as well as in the crenulations on their upper side 

 being less distinct, and less regular ; in the shell being much 

 thicker, and its posterior side being considerably shorter. 



Found in the Wharf, near Ottley; the Aire, near Keighley; 

 the Wharf, near Bolton Bridge; and the Don, near Sheffield. 

 3. Unio rostrata, pi. XX, f. 1, 2. 



Unio rostrata, Kokeil Museum; Alder, Mag. Zool. and Bot., 

 II, p. 118; Lamarck, An. San. Vert., VI, pt. 1st, p. 77, No. 

 31 ; Brown, lUust. Conch., p. 82, pi. 32, f. 9, 10, 11, 12; Unio 

 elongatata, Pfeiffer, II, p. 35, pi. 8, f. 5, 6 ; Unio pictorum, 

 var., Nilsson, I, p. 118, ff; lb., Ill, p. 23, ff; Rossmassler, 

 VI, p. 55, pi. 29, f. 409. 



Shell greatly elongated transversely ; umbones very flat, 

 placed near the anterior side ; hinge line but very little 

 arcuated, as far as the extremity of the lateral teeth, from 

 whence it suddenly slopes both above and below, termina- 

 ting in a somewhat rostrated form ; basal line nearly parallel ; 

 primary teeth rather thick, and serrated at the edge ; lateral 

 teeth long, and considerably elevated ; anterior muscular im- 

 pression moderately sized, and deeply sunk ; below the 



