106 Unio distortus (Bean) and Alasmodon vetustus (Brown). 
them. In the fossils there is also the same deep pit-like cavity 
immediately in front of the umbo, as is observable in the 
eroded example of Margaritana just mentioned, which seems. 
to point to the presence of a definite pseudocardinal tooth here 
as in Margaritana. Whether the fossils possess posterior 
lateral lamellz or not, Iam unable to say at present, as I have 
been unable, so far, to procure casts of these fossils in order to 
observe this feature. In the Manchester Museum, however, 
there is an example of a Unio from the Estuarine Beds of York- 
shire, which is undoubtedly a juvenile form of Unio distortus, 
and shews the resemblance to a recent Margaritana in a remark-- 
able manner. It forms part of the Williamson collection, and, 
as will be seen by the photograph on pl. X. (fig. 2), is remark- 
ably well-preserved, exhibiting only slight traces of decortica- 
tion in the umbonal region. It is, fortunately, quite uncom- 
pressed and distinctly shows a tendency towards alation at the 
posterior dorsal margin—a feature met with in recent forms 
which do not possess posterior lateral lamellz, such as Anodon, 
Margaritana, etc. In Margaritana, however, rudimentary 
posterior lateral lamelle are often present in young examples. 
Casts of Unioniform shells have been noticed in the Estuarine 
Bed of Yorkshire, by T. Wright* and John Leckenby,* but 
no reference to dentition is contained in their papers. Hudles- 
ton, however, in his paper on the Yorkshire Oolites*, states. 
in a footnote on p. 318, that Leckenby obtained casts of the 
interior of a Unio-like shell, which shewed the hinge to be 
edentulous, the shells being, therefore, referred to Anodon. 
This fact is of some interest, but it appears to me possible that 
indications of pseudocardinals, which are usually rather in- 
conspicuous in casts of recent Margaritanas, might easily have 
been overlooked in these casts, and deductions made mainly 
on the absence of posterior lateral lamellae. That Bean’s 
type is not an Anodon, is conclusively demonstrated to my 
mind by the deeply impressed character of the anterior ad- 
ductor scar, as well as, to a lesser extent, by the presence of 
so robust a hgament. Moreover, Anodons seem never to be 
so deeply eroded as Unios; the beaks, or umbones, too, are 
much flatter. 
That distortus possessed some sort of pseudocardinal teeth 
is suggested by the presence of a well-defined lunule, a feature 
which is met with in all recent members of the genus Unzo, 
but which is almost, or entirely absent in edentulous forms, 
such as Anodonta. It might be argued that no lunule is 
present in Margaritana margaritifera, but such is not the case, 
5.’, vol. XVI. (1860),.pp. 30, etc. 
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