12] 
UNIO crassissimus. 
TAB. CLIII. 
Spec. Cuar. Ovate, transversely undulated or im- 
bricated; beak recurved, acute; posterior 
side short, round; anterior side obscurely 
subcuneiform ; shell very thick. 
Se 
Tue hollow below the beaks is deep in consequence of 
the beaks being much incurved towards the posterior side ; 
the cartilage slope is rounding and the front nearly 
straight: length two-thirds of the width. The shell is 
convex outside and in thickness equal to the internal 
depth; the hinge is particularly massive. 
Mr. Wood having figured Mya crassa as a thick shell 
in tab. 20 of his General Conchology, I must now 
use the superlative degree of the word to this Mya 
of Linn. but Unio of later authors, and thus, in some 
measure, designate the species. I have had the speci- 
mens by me for some years, favoured by Dr. Sutton, of 
Norwich, and was pleased to find sufficient of the hinge 
to determine the Genus, which has puzzled Mr. Park- 
inson, who has, not without doubting, made it a Donax. 
This Gentleman observes they are usual in Gloucester- 
shire and Wiltshire, near Bath, sometimes in the Lias 
Clay. Dr. Sutton gave me many specimens of Fossil 
shells as British, without localities, among which are 
several of this species, all formed of Carbonate of Lime : 
upon opening one of the pairs the hinge was found con- 
cealed among equiaxed crystallizations (British Mine- 
ralogy tab 13) beautifully showing the manner of modi- 
fication, &e. 
