Mr. J. Lycett on Perna quadrata. 429 



and from the figure in the ' Mineral Conchology,' so that tlie 

 identity of these speeimcns 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 Pernae is therefore very great ; its general aspect is in 

 fact that of an Inucerumus, more especially of the subinvolute 

 forms of the latter genus, from which it is distinguished only by 

 the anterior excavation and a])crture ; 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, w'hose relation to the typical PerruB may be 

 compared with that which certain aberrant species of the kindred 

 genera Inoceramus and Gervillia bear to their res])ective typical 

 groups. Of the Jurassic forms more especially may be instanced 

 the large Gervillia Hartmanni and G. tortuosa, compared with the 

 flattened subequivalve 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 wdth 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 ow^n specimens are from the freestone portion of the Inferior 

 Oolite near Nailsworth, from a quarry in Woodchestcr Park, 

 w'orked 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 afi'ording an 

 additional instance of that general resemblance between the 

 Testacea of the Cornbrash and the Inferior Oolite, which has 

 been noticed by Professor Buckman 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 qua- 

 drata 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 limestone w^ith Nerinaas which 

 replaces it in the valley of Nailsworth. 



