portion 



angular. 



(4) 



(5) 



23 . 



1 1 1)11)1 



40 . 



22 „ 



PBRNA. 93 



forming an obtuse angle with the hinge-line. Umbonal portion angular. Apical 

 angle small. Surface with growth-lines. 

 Measurements .- 1 



(1) (2) (3) 



Length . 35 . 34 . 24 



Height . . 73 . 69 . 49 



(1 — 3, 5) Cambridge Greensand. 



(4) Lower Gault, Folkestone. 



Affinities. — This species is closely allied to P. Ricordeana (see above), but 

 appears to differ (1) in being relatively shorter and higher; (2) in its more obtuse 

 postero- dorsal angle ; (3) in the less prominent nmbones ; ( !•) in the anterior 

 marginal parts (near the byssal opening) being less depressed; (5) in the smaller 

 size of the shell. 



Remarks. — This species occurs commonly in the Cambridge Greensand in the 

 form of internal casts ; these differ in outline from specimens with the shell pre- 

 served on account of the fact that the postero-dorsal part is missing — probably 

 owing to the two valves being in contact or almost in contact at this part. Even 

 when the shell is preserved, as in specimens from the Gault, some portion of the 

 postero-dorsal margin is frequently missing. 



Types. — From the Albian of Avocourt (Meuse) and Escragnolles. 



Distribution. — Cambridge Greensand (derived, internal casts). Lower Gault of 

 Folkestone. 



Perna oblonga, Seeley, 1861. Text-figure 10 a. 



1861. Perna oblonga, H. G. Seeley. Ann. Mag. Nat. Hist., ser. 3, vol. vii, p. 121, 



pi. vi, fig. 6. 



Remarks. — This species is known only from internal casts, and apparently 

 differs from P. Rauliniana in having a larger apical angle and in the smaller angle 

 formed, by the hinge-line and the posterior margin. Some of the specimens, how- 

 ever, agree very closely with the figure of a cast of P. Rauliniana given by Pictet 

 and Campiche (pi. clx, fig. 2). 



Seeley compared P. oblonga with P. subspathulata, Reuss, 2 and P. lanceolata, 

 Geinitz (see below). It is distinguished from the former by its relatively greater 

 height, and from the latter by its larger apical angle. 



1 The height in this case is measured from the umbo to the postero-ventral margin, and the 

 length is taken at right angles to the height. 



- ' Die Verstein. der bohm. Kreideformat.,' pt. ii (1840), p. 24, pi. xxxii. figs. 16, 17. 



