104 
ON UNIO DISTORTUS BEAN, AND ALASMODON 
VETUSTUS BROWN, FROM THE UPPER ESTUA- 
RINE BEDS OF GRISTHORPE, YORKS. 
J. WILFRID JACKSON, F.G.S., 
Manchester Museum. 
(PLATES Ix. and x.). 
In 1843, Captain Thomas Brown described a number of fossil 
fresh-water mussels* under the generic name of Pachyodon, 
amongst them being two species found by Dr. Fleming in 
oolitic strata at Gristhorpe, Yorkshire. These were described 
as P. hamatus and P. vetustus, the latter being regarded by 
Brown as possibly belonging to the genus Alasmodon. 
Since Brown’s time these two species, presumed to be in 
the Manchester Museum, appear to have been entirely lost 
sight of, like many others he described. Nor do they appear 
to have ever been quoted except by Morris, in the second 
edition of his “ Catalogue of British Fossils.’ It will be of 
interest, therefore, to know that whilst overhauling a number of 
miscellaneous fossils in the Manchester museum some time ago, 
I had the good fortune to discover a specimen labelled ‘ Alas- 
modon vetustus, Gristhorpe,’ and later on I found another 
Unio-like shell possessing a label, ‘ Gristhorpe Bay. 
These specimens, I fcund on reference, compared tolerably 
well, both as to size and markings, with the figures and des- 
criptions given by Brown in the above-mentioned magazine, 
and in his ‘ Fossil Conchology.’t 
His figures certainly represent shells in much better con- 
dition than those under discussion, but there appears to be no 
doubt as to the shells in question being those selected by Brown 
as his types, his figures being restorations based upon characters 
exhibited by the specimens. Further confirmatory evidence in 
the case of vetustus, is also afforded by the fact of the specimen 
possessing the irregular break at its posterior extremity, 
depicted by Brown in his illustration of the species. 
In Brown's later work this species is referred to as ‘ Alas- 
modon vestustas—the Ancient Alasmodon,’ which is obviously 
a mistake, as the latin word for ‘ancient’ is ‘ vetustus,’ as 
given in the original description. On the other hand, ‘ hama- 
tus’ is corrected to ‘ Unio humatus—the Buried Unio,’ and 
the geological horizon altered to ‘Coal Shale at Gristhorpe 
Bay,’ instead of “ Oxford Clay,’ as originally stated. 
Brown’s description of vetwstus is, in its main details, fairly 
accurate. The only exception to be made is with regard to 
* “Ann. Mag. Nat. Hist.’, vol. XII., pp. 395, etc. 
Tt ‘London,’ 1849, pp. 179 and 181, pl. 72x., figs. 18 and 19. 
Naturalists 
