1909.] NATURAL SCIENCES OF PHILADELPHIA. 537 



dorsal slope near the beaks, when not worn off. Beaks low, between 

 the anterior fifth and sixth of the length; when perfect the tip is acute 

 and sculptured with a series of short folds on the posterior ridge. The 

 interior is fleshy-purplish, iridescent posteriorly; nacre in the anterior 

 half is very thick, posteriorly thin. Cavity of the beaks deep and 

 angular. The pseudocardinal teeth are stout, otherwise formed as in 

 L. signata; laterals curved. 



Length 54, alt. 33, diam. 19 mm. 

 " 54, " 30, " 18 " 

 " 50, " 30, " 18 " 



Valles River two miles above Mecos, in a muddy bank just above 

 rapids. 



This species is closely related to L. signata, but the valves are thicker 

 and heavier, the teeth are heavier, and the proportions differ. It is 

 apparently related to L. aztecorum (Phil.), 1 but that is described as 

 thin, "valvulis satis tenuibus," whereas in L. undivaga the valves are 

 quite heavy and strong for a Lampsilis of this size. 



The nacre is nearly white in a few examples. It abruptly becomes 

 thick in the posterior half of the shell. Young shells are conspicuously 

 rayed with green. 



One young shell from the Valles River at Valles (pi. XXVII, fig. 5), 

 seems to belong to this species, though I am not quite sure of the 

 identity. It is 41 mm. long, yellowish, nearly covered with green rays, 

 the nacre is pale purple, with a wide darker purple margin posteriorly. 



Lampsilis novileonis n. sp. PI. XXVII, figs. 1, 2. 



The shell is oblong, rather compressed, the upper and basal margins 

 slightly and about equally arcuate, anterior end rounded, posterior 

 end sloping above, truncate and somewhat biangular below, beaks 

 at the anterior two-sevenths of the length. The surface is convex, 

 without angle or ridge bounding the posterior slope, not very glossy, 

 smoothish for the greater part, but on the posterior slope there is 

 some oblique corrugation in the upper part of the disk. The epidermis 

 is yellow, sparsely marked in the posterior half with narrow green 

 rays. The very low beaks are eroded in the type specimen. The interior 

 is white, thicker anteriorly, very gradually becoming thinner at the 

 posterior end, where opalescent tints are beautifully produced. The 

 cavity of the beak is rather deep. The lateral teeth are short and 

 rather widely separated from the stout cardinals, which are double 

 and about equally prominent in the left valve. 



1 Unio aztecorum Philippi, Abbildungen und Beschreibungen neuer oder wenig 

 bekannter Conchylien, III, pi. 6, fig. 2. 

 35 



