66 CRETACEOUS LAMELLIBRANCHIA. 



Pteria (Pseudopteba) haldonensis, sp. nov. Plate IX, figs. 5, 6 a, b, 7, 8 a-c, 9, 10. 



Description. — Shell of moderate size, triangular, very oblique. Anterior margin 

 slightly convex, forming a rounded angle with the postero-ventral margin. Umbo 

 pointed, acute, near the anterior extremity. Apical angle about 43°. 



Left valve very convex, with a sharp carina extending from the umbo to the 

 postero-ventral angle. The part of the valve in front of the carina is bent 

 sharply downwards along its whole length, and is ornamented with from ten 

 to eighteen slender, linear ribs, which are separated by broad flat interspaces. 

 The number of ribs increases with age owing to the intercalation of new ribs 

 in the interspaces. The space between the carina and the first rib, and 

 sometimes also between the first and second rib, is greater than the space 

 between the ribs near the middle of the anterior part of the valve. Minute 

 spiny projections are present on the ribs in well-preserved specimens. A similar 

 but rather stronger rib, also with spiny projections, occurs on the carina. 

 Behind the carina two short ribs, extending from near the middle to the margin of 

 the valve, are sometimes seen. The larger part of the valve behind the carina is 

 flattened and smooth except for numerous, slightly curving growth-ridges, which 

 are continued on to the posterior ear, and are sometimes seen in front of the carina, 

 where they may become more prominent. 



Anterior ear small, with rounded margin, indistinctly separated from the 

 remainder of the valve, ornamented with radial ribs similar to those on the 

 adjoining part of the valve. 



Posterior ear compressed, very large, separated from the remainder of the 

 valve by a very shallow depression. Growth-ridges concave and parallel with 

 the posterior margin. 



Right valve not seen. 



Measurements : 



(i) 

 Height (oblique) . 28 

 Length of hinge-line x 19 



(1 — 5) Upper Greeiisand, Haldon. 



Affinities. — This species is closely allied to Pterin (Pseudoptera) raricosta (Reuss), 2 

 from the Gosau Beds of St. Wolfgang (Salzburg), but is distinguished by the 

 smaller obliquity of the shell, by the angle formed by the anterior and postero- 



1 This measurement is approximate only, since the posterior wing is usually imperfectly preserved. 



- Reuss, ' Char. d. Kreideschicht. in den Ostalpen,' etc. (Denkschr. d. k. Akad. Wissensch. Wien, 

 Math.-nat. CI., vol. vii, 1854), p. 147, pi. xxviii, hg. 16; K. A. Zittel, 'Die Bivalven d. G-osaugeb.' 

 (Ibid., vol. xxv, pt. ii, 1866), p. 90, pi. xiii, fig. 6; A. glabra, Greinitz, 'Das Elbthalgeb. in Sachsen ' 

 (Pakeontographica, vol. xx, pt. i, 1873), p. 208, pi. xlvi, fig. 7, and pt. ii, pi. xi, fig. 2 ? ; Notling, ' Die 

 Fauna d. baltisch. Cenoman.' (Palaeont. Abhandl., vol. ii, 1885), p. 22, pi. hi, fig. 9. 



(2) 



(3) 



(4) 



(5) 



26 , 



, 25 



. 23 , 



18 mm, 



16 , 



, 18 



. 15 



12 



