Unio distortus (Bean) and Alasmodon vetustus (Brown). 105 
the anterior end of the shell, which is given as sub-acute. An 
examination of the fossil, however, revealed the fact that a 
large portion is broken away from the antero-ventral margin, 
thus giving to the shell its sub-acute appearance. <A con- 
tinuation of the growth-lines visible near the ventral border, 
proves the shell to have been rounded anteriorly, and a restora- 
tion of this end with plasticine makes this point quite clear. 
Having this specimen with me when on a visit to the British 
Museum recently, I took the opportunity of comparing it 
with the type specimen of Unio distortus Bean,* and was at 
once struck by the remarkable similarity of the two specimens. 
Brown’s species ‘vetustus’ is rather more convex than Unio 
distortus, and is preserved in a somewhat different matrix, but 
the difference between the two forms—probably only one of 
condition—appeared to me to be so slight that I had no hesita- 
tion in concluding that they w ere one and the same species. 
This being so, Brow n’s name ‘ vetustus,’ although perhaps a 
much better name than ‘ distortus,’ must now fall, and become 
a synonym, as Bean’s name antedates it by some seven years. 
The distorted and crushed condition of Bean’s type of 
Unio distortus, and the somewhat imperfect state of Brown’s 
shell, does not permit of a proper diagnosis being made of 
their chief characters, but both superficially present a most 
remarkable resemblance to the recent Pearl-mussel (Mar- 
garitana margaritifera). 
From the condition of the umbonal region in both specimens, 
it is quite evident that this part of the shell suffered considerable 
erosion, as is the case with recent shells of Margaritana. We 
may reasonably infer that this condition was brought about by 
similar means to those obtaining at present, viz.—the presence 
of humic and carbonic acids in water containing a low percen- 
tage of calcium carbonate, or none at all. In the fossil cases, 
the humic acid would, in all probability, be derived from the 
numerous forms of vegetation met with in the Estuarine Beds 
in which the shells occur, while the absence of any large amount 
of calcium carbonate might be accounted for by ihe fact of 
the catchment basin of the river being situated on rocks con- 
taining no soluble salts of lime. 
Both the fossil examples present the same features as a well- 
eroded specimen of Margaritana margaritifera from the R. 
Lune, above Lancaster [pl. X., fig. 1], with which they were 
compared. In each case the back of the anterior adductor 
scar is clearly visible owing to excessive decortication at 
this end of the shell. The umbones, too, are widely separate, 
exposing what now remains of the umbonal ligament between 
* Described and figured from a much-crushed specimen in ‘ Mag* 
Nata EIStn. VOl. exes (a 836), Ps 370) text Hie .. 5B. 
gir Feb. 1. 
