Mr. J. Lycett on Perna quadrata. 427 



XXXVII. — On Perna quadrata, Sow. By Joux Lycett, Esq.* 



The present notice of a fossil shell, which has already been 

 figured and described, requires some explanation. 



The Perna quadrata of the ' Mineral Conchology,^ t. l-DO. f. 2, 

 represents a very inequivalve Perna ; but as the specimen figured 

 appears to have been somewhat imperfect, and as it was placed 

 by the artist in an unfortunate position and is seen only from 

 a single point of view, it affords a veiy insufficient aid to a de- 

 scription which is substantially correct as far as it refers to the 

 only specimen which appears to have been at the disposal of the 

 author. With such an illustration it will not aflford surprise to 

 find that Phillips in his Geology of Yorkshire, t. 9. f. 24, repre- 

 sented a second and very difierent shell under the name of Perna 

 quadrata ; and as the latter w'ork contains no descriptions of spe- 

 cies, Goldfuss was in tui-n also misled to figure a third Perna 

 distinct from both the others under the same name (Petrefacta, 

 t. 108 ); upon the same plate however is Perna rugosa, which is 

 identical with the quadrata of Phillips ; the latter is an equivalve, 

 squamous, thick shell, well known to collectors of British Inferior 

 Oolite fossils. There is another inducement to acquire a correct 

 knowledge of the original Perna quadrata, inasmuch as the spe- 

 cies is a very remai'kable one, constituting a departure from the 

 typical forms of the Pernce, and approaching to others of the 

 kindred genus Inoceramus. The diagnosis of ths shell in the 

 * Mineral Conchology ' is as follows : — " Quadrilateral, one side 

 shorter than the other three ; valves gibbose, unequal, the shorter 

 side very concave, bounded by two obtuse carinae." 



The figure in the ' Mineral Conchology ' has the right or 

 flattened valve facing the spectator ; the contour of the larger or 

 convex valve therefore is not seen : the shell is not placed upright 

 upon the page, the lower border forming the right-hand side of 

 the figure : even the outline is not perfect, as there seems to be a 

 portion of the lowxr border wanting, and thus forming an angle 

 at its anterior extremity, which would be rounded were that part 

 entire. 



The typical Pernce are equivalve or subequivalve ; their fibrous 

 tests are squamous externally, and acquire great thickness with 

 advance of growth ; the valves are so much flattened, that their 

 attenuated apical extremities have not more thickness or con- 

 vexity than the corresponding parts of the Pinna and Mytili ; 

 the hinge-plate is always broad, the greater length of the valve 

 being always perpendicular to, or in the opposite direction to 

 the line of the hinge-plate. Perna quadrata, on the contrary, 



* Read to the Cotteswold Naturalists' Club, January 30th, 1855. 



28* 



