BIVALVIA. 25 



A somewhat oval, depressed, and thin shell, with the hinge margin oblique, and rather 

 prominent umbones ; surface irregularly undulated. 



A shell presenting considerable resemblance in form to I. amydaloides, Goldf., but 

 we have only been enabled to compare it with casts of that species which is found in the 

 Lias of Germany. 



Locality. Stonesfield, Oxfordshire, where it occurs but rarely. 



Pern a, Brugiere, 1791. 



Gen. Char. Shell flattened somewhat irregular, with terminal depressed umbones, 

 hinge linear marginal, with numerous parallel ligamental pits; byssal sinus anterior placed 

 a little beneath the umbones, and slightly gaping, its margins thickened; muscular im- 

 pression oval and situated rather posteriorly; texture of the shell fibro-laniellar. 



Pern a rugosa. Tab. Ill, fig. 1. 



? Pern a rugosa, Goldf. Petref. 2. t. 108, f. 2. 



Testa subquadratd, complanatd, rugosa, tenui, umbonibus acutis, lined cardinali 

 horizontali ; laminis superficial scabris irregidaribus. 



Shell subquadrate, flattened, rugose, thin, with acute umbones, hinge line horizontal 

 and of moderate length ; laminae of the surface rough and irregular. 



The general figure possesses a considerable resemblance to P. rugosa, Munst. (Goldf. 

 Petref., 1. 108, fig. 2,) our specimen is, however, more compressed, and the umbones less 

 prominent, the test is also somewhat thin. The tenuity of the test affords a striking contrast 

 to several massive Inferior Oolite species, whose general contour is not very dissimilar. 



Locality. Minchinhanipton Common, where it occurs very rarely in the bed of coarse 

 planking. The P. quadrata, Phil, is probably identical with this species. 



Lima, Brug., 1791. 

 Plagiostoma, Sp., Sow. 



Gen. Char. Shell subequivalve, inequilateral, oblique, aurited, hinge margin oblique, 

 thickened within, forming a transversely flattened plate, in which and beneath the umbones 

 is a triangular depression, destined to receive a ligament which is partly internal. 1 



1 In an interesting communication read before the Linnean Society of alvados, (December, 1830,) 

 Professor E. Deslongchamps stated the general reasons for uniting the species of Plagiostoma to Lima. 

 In this memoir, not yet we believe fully published, M. Deslongchamps described seventeen species of Lima 

 from the Jurassic strata of Calvados, and arranged them under four sections : — 



1. Margins of the valves entire, not serrated, luuule distinct: L. gigantea, heteromorpha. 



2. Margins of the valves sinuato-dentate, lunule distinct: L. sulcata, variabilis, radiata, punctata. 



3. Margin of the valves serrated, lunule distinct : L. elliptica, lucida, pulckella, uniaurita, t>j/n>*, 



IcBvis, semistriata. 



4. Margins of the valves sinuato-angnlar, no lunule : L. alternans, duplicates, gibbosa, exigua. 



4 



