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 quadrata presents to the 

 typical Perna 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 Perna 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 3| inches, 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 Nerinaas which replaces it in the valley of Nailsworth. 



