214 Jackson: A New Species of Unio 
disjointed with occasional short ridges intercalated between 
the longer ones. On another specimen (fig. 3) these shorter 
ridges are connected with the longer ones by means of an 
upward loop. Unlike modern European Unios the sculpture 
is not confined entirely to the nepeonic stage, but is continued 
well into the neanic stage, more especially on the posterior 
slope. This sculpture is quite unlike the ordinary concentric, 
doubly-looped variety met with on modern European Unio- 
nid ; but appears to me to be more nearly, though not abso- 
lutely, related to the zigzag-radial type of Asiatic forms— 
Nodularia, Pseudodon, etc., whose areas are crossed by similarly 
disposed ridges. Unfortunately, no examples have been so far 
found which are sufficiently preserved at the anterior end to 
show the sculpture on that area. 
Several of the casts, after careful treatment, revealed the 
muscular impressions, especially the anterior ones, very 
clearly (fig. 1). Compared with modern forms, the positions 
occupied by the anterior pedal protractor and retractor scars, 
in their relation to the adductor scar, are not markedly different, 
excepting perhaps that the retractor appears to occupy a 
slightly lower position than in most modern examples. 
After careful development along the hinge-line of some of 
the casts I was successful in exposing some interesting evi- 
dences of well-marked pseudocardinal teeth and _ strongly- 
developed poster-lateral lamellae. The latter, two in the left 
valve, and one in the right, are long and fairly straight, ex- 
tending from behind the umbo almost to the postero-dorsal 
angle. The pseudocardinals consist of one large tooth in the 
right valve, which appears to be clasped by two processes in 
the left valve. The type of dentition is not unlike that of 
Unio batavus, Margaritana margaritifera, etc., etc., but not 
like that developed in Unio pictorum, U. tumidus, etc., where 
the pseudocardinals are distinctly lamellar in character. More- 
over all the specimens of these latter species which I have 
examined possess ‘wo distinct anterior lamelle 71m each valve. 
One of the casts (fig. 1) shows what appears to be an ab- 
normality in the anterior dentition of the right valve. Here a 
well-marked triangular sheli-like process is present, bounded 
on its two lower sides by deep grooves. Either one of the 
pseudocardinal teeth of the left valve was unusually large, or 
the anterior portion of the hinge-line has been crushed down 
and thus somewhat distorted the dentition. The latter seems 
the most probable, as none of the other casts I developed show 
this peculiarity. 
Judging from the impressions of the growth-lines and beak- 
sculpture present on some of the casts, the shell was evidently 
thin-tested. 
Naturalist, 
