120 Mr. J. Lycett on Perna quadrata. 
specimens of the same species, the result not only of different 
stages of growth, but also of individual peculiarities. All the 
specimens of Perna quadrata differ more or less from each other 
and from the figure in the ‘ Mineral Conchology,’ so that the 
identity of these specimens with Sowerby’s shell has not been 
ascertained without the examination of a considerable number of 
examples. The contrast which Perna guadrata presents to the 
typical Perne is therefore very great; its general aspect is in 
fact that of an Inoceramus, more especially of the subinvolute 
forms of the latter genus, from which it is distinguished only by 
the anterior excavation and aperture; but as this feature is one 
only of subordinate value when viewed singly, there would seem 
to need the addition of some other distinctive features ere we 
are enabled to affirm the clear generic separation of Perna and 
Inoceramus. The oblong flattened figure of the smaller valve 
and the thinness of the test might cause it to be mistaken for a 
Crenatula when the hinge-plate is not exposed. Perna quadrata 
may therefore be regarded as the type of a group of inequivalve 
transverse shells, whose relation to the typical Perne may be 
compared with that which certain aberrant species of the kindred 
genera Inoceramus and Gervillia bear to their respective typical 
groups. Of the Jurassic forms may be instanced the large Ger- 
villia Hartmanni and G. tortuosa, compared with the flattened sub- 
equivalve species of the same genus ; in these and other instances 
the inequality of the valves becomes more marked with advance of 
growth. The very perfect preservation of the hinge-plate, together 
with the condition of the tests of associated bivalves, forbids the 
supposition that the thinness of the test has been produced by 
the removal of thick nacreous layers from the inner surface. 
Dimensions.—Length of our largest specimen in the direction 
of the hinge-line 5; inches, height 33 ches, convexity of the 
larger valve 2 inches. 
Geological position and localities. —The specimen figured in the 
‘Mineral Conchology’ is from the Cornbrash of Bulwick ; our own 
specimens are from the freestone portion of the Inferior Oolite 
near Nailsworth, from a quarry in Woodchester Park, worked for 
the purposes of the Roman Catholic Monastery ; specimens have 
also been obtained in the freestone quarries of Scar Hill in the 
parish of Minchinhampton, thus affording an additional instance 
of that general resemblance between the Testacea of the Cornbrash 
and the Inferior Oolite, which has been noticed by Prof. Buck- 
man in a paper on the Cornbrash of Cirencester, and which forms 
a part of the Proceedings of the Cotteswold Naturalists’ Club. 
The exact position of Perna quadrata in the freestone is the two 
uppermost beds of that rock, immediately underlying the Oolite 
marl, or in its absence, the cream-coloured hard argillaceous lime- 
stone with Nerineas which replaces it in the valley of Nailsworth. 
