94 



CRETACEOUS LAMELLIBRANCHIA. 



Rounded depressions which are found commonly on the casts of this and some 

 other species of Perna from the Cambridge Greensand are regarded by Seeley as 

 evidence of the occurrence of pearls. 



A specimen from the Gault (Plate XII, fig. 10) resembles P. oblonga, but is 

 more convex between the umbo and the postero-ventral extremity. 



Type. — In the Sedgwick Museum. 



Distribution. — Cambridge Greensand (derived). 



Fig. 19.— Perna. Cambridge Greensand (derived). Internal casts. Sedgwick Museum. Natural size, 

 except fig. e. — a. Perna oblonga, Seeley. The Type. — b, c, Perna, sp. (referred to P. lanceolata, Geinitz, 

 by Seeley). — d, Perna semielliptica, Seeley. The Type.— e, Perna, sp. (regarded by Seeley as a variety 

 of P. lanceolata, Geinitz) x J. — f, Perna, sp. (referred to P. subspathulata, Eeuss, by Seeley). 



Perna semielliptica, Seeley, 1861. Text-figure 19 d. 



1861. Perna semielliptica, H. G. Seeley. Ann. Mag. Nat. Hist., ser. 3, vol. vii, 



p. 121, pi. vi, fig. 7. 



Remarks. — This is distinguished from P. Bauliniana (p. 92) by its relatively 

 greater length, larger apical angle, and more rounded outline. 



