Some new Upper Greensand Bivalves. 121 



those of the hinge. The specimens appear quite adult, and 

 had the lips, which were smooth, much thickened. 

 This remarkable species, which approaches N. Vibrayeana 

 (B'Orb.), is distinguished by its quadrilateral figure. 

 Very rare. Collected by Mr. W. Farren. 



Perna oblonga. PI. VI. fig. 6. 



Shape elongated, rectangular, narrow, very moderately com- 

 pressed. An inflation, often prominent, occurs along and 

 near to the upper part of the anterior side. Posterior extre- 

 mity rounded, rarely subangular. Hinge-line short and 

 straight, making with the anterior side (not with the beaks) 

 an angle of about [7o° to) 90°, and with the posterior side 

 some greater angle. The posterior side is generally gently 

 curved, causing the shell to be widest in the middle. The 

 beaks are generally very moderately curved outwards, some- 

 times not at all. The muscular impression is midway between 

 the hinge and base, and rather large. 



This species is no exception to the general variableness of 

 Perna ; but there is never any difficulty in recognizing the spe- 

 cific type. In one specimen both pairs of sides are parallel. 



This species is intermediate between P. subspathulata (Reuss) 

 and P. lanceolata (Geinitz), combining with the growth of the 

 former the width of the latter. It is distinguished from P. lan- 

 ceolata chiefly by its large apical angle and perpendicular form, 

 and from P. subspathulata by its elongated figure. 



Not uncommon. Cambridge and Ashwell. Coll. University ; 

 J. Carter, Esq. ; Mr. W. Farren. 



Perna semielliptica. PI. VI. fig. 7. 



Shape semi-elliptical, with the anterior side of the base rounded. 

 Anterior side straight or hollowed in the byssal region. 

 Hinge-line short, straight, uniting imperceptibly with the 

 curve of the posterior side, and making with the anterior 

 side a large angle (about 75° to 90°). In the half-grown state 

 it is moderately inflated ; but aged specimens have a very 

 compressed aspect, owing probably to internal thickening of 

 the shell. It is generally a fourth longer than wide, and has 

 its greatest width midway between the hinge and base. 

 This species is intermediate between P. Rauliniana of D'Orb. 

 and P. subsjjathulata (Ucuss). From the former it is easily di- 

 stinguished by its greater proportionate width and greater apical 

 angle ; and from the latter by the short hinge-line curving into 

 the posterior side. Most of the half-grown specimens are 

 Ann. ^ Mag. N. Hist, Ser.3. Vol.^iu 9 



