PTERIA. 61 



known, the characters and affinities of this "species " cannot be determined. The 

 left valve has a length of 6 mm. 



Types. — In the Sedgwick Museum, Cambridge. 



Distribution. — Totternhoe Stone (zone of Rolaster subglobosw) of Burwell. 1 



Pteria (Oxytoma) tenuicostata (Bomer), 1841. Plate VIII, figs. 17 a-d, 18, 



19a, b, 20a, b, 21 a, b, 22, 23. 



1841. AvicuLA lineata, F. A. Burner. Die Verstein. d. uord-deutsch. Kreidegeb., 



p. 64, pi. viii, fig. 15 (A. tenuicostata on 

 pi. viii). 

 1850. — sublineata, A. d'Orbigny. Prodr. de Pal., vol. ii, p. 249. 



1869. — tenuicosta, F. J. Pictet and G. Campiche. Foss. Terr. Cret. Ste. 



Croix (Mater. Pal. Suisse, ser. 5), 

 p. 73. 

 1878. ■ — tenuicostata, J. F. Blake. Proc. G-eol. Assoc., vol. v, p. 259. 



1882. — — H. Schroder. Zeitschr. d. deutsch. geol. Gesellsch., 



vol. xxxiv, p. 271. 

 1888. A. Peron. L'Hist. du Terr, de Craie, p. 153, pi. i, 



figs. 11, 12. 

 1904. — A. W. Bowe. Proc. Geol. Assoc, vol. xviii, p. 266. 



Non 1845. A. d'Orbigny, in Murchison, de Verneuil, and Keyser- 



ling. Geol. de la Eussie d'Europe, 

 vol. ii, p. 490, pi. xliii, figs. 5 — 7. 

 — 1854. — lineata, J. Morris. Cat. Brit. Foss., ed. 2, p. 163. 



Description. — Shell obliquely oval, usually longer than high. 



Left valve moderately convex ; with evenly convex margins, except the postero- 

 dorsal, which is slightly concave. Ears large, the anterior indistinctly limited, and 

 with its outer angle rectangular or slightly obtuse. Posterior ear longer and more 

 distinctly limited than the anterior, with the dorsal portion extended and wing- 

 like. 



Ornamentation of left valve consists of numerous (sometimes as many as 100) 

 narrow, well-marked, evenly rounded ribs separated by broad, flat interspaces. 

 The anterior ribs are slightly less prominent than the others ; those near the pos- 

 terior border are often closer together. At the margin of the valve the ribs usually 

 1 Another specimen from the same locality and horizon was described by Etheridge as Avictda 

 filata (Penning and Jukes-Browne, 'Geol. Camb.,' p. 144, pi. ii, fig. 3). I am unable to accept the 

 generic position assigned to this species by Etheridge ; it may be an Ostrea, but appears to be closely 

 allied to the shell described as Anomia subradiata by lleuss ('Die Verstein. der bohm. Kreideformat.,' 

 pt. 2, 1846, p. 45, pi. xxxi, figs. 18, 19). The type and three other specimens of Avicula filata are in 

 the Sedgwick Museum, Cambridge. 



