TURNUS. 233 



Martesia? kotuxda {Soirn-hi/), 18-50. Plate XXXVIII, figs. 12 (t—c, 18. 



1850. Teredo eotundtjs, J. de C. Sowei-hy iu F. Dixon. Geol. Sussex, p. 340 (p. 



382, ed. 2), pi. xxviii, figs. 27, 28. 

 1854. — — /. Morris. Cat. Brit. Foss., ed. 2, p. 227. 



1897. Martesia ? rotunda, H. Wnixh. Quart. Jouru. Geol. Soc, vol. liii, 



p. 393, pi. xxviii, figs. 15 — 18. 



Description. — Shell small, ovoid, inflated, with rounded outline. Anterior gape 

 large, apparently closed by callus. Umbones sub-median, curved inwards and 

 forwards. A strong groove and a corresponding internal rib pass from the umbo 

 to the ventral margin. Near the dorsal margin a strong, narrow internal rib passes 

 backward from the umbo but does not reach the posterior margin. Ornamentation 

 posterior to the groove consists of concentric ribs and of two radial ribs imme- 

 diately behind the gi'oove; anterior to the groove are a few regular radial ribs, and 

 concentric ribs parallel to the margin of the anterior gape. Length, 7 mm. ; 

 height, 5'5 mm. ; thickness, 5 mm. 



Bemarlis. — I have seen only three casts of the interior and two of part of the 

 exterior of this species, so that at jjresent it is difficult to determine satisfactorily 

 its generic position. 



Type. — The type came from the Chalk of Kent, l)ut cannot noAV l)e found. 



Distribiitiuu. — Chalk Rock of Cnckhamsley (Berkshire), and of AVhyteleafc 

 (Surrey). 



Genus— TuR^vn, W. M. GahJ,, ISGl. 

 (' Geol. Surv. California,' Pala;outology, vol. i, p. 145.) 



TuRNUs Dallasi {Wall-cr), 1866. Plate XXXVIII, figs. 14 a, h, 15. 



18(3(5. Pholas Dallasi, /. F. Walker. Ann. Mag. Nat. Hist., ser. 3, vol. xviii, 



p. 38(5, pi. xiii, figs. 3, 4. 



De.^crij)ti()n. — Shell ovate, short, inflated in front, wedge-like behind. Margin 

 of the anterior gape oblirjue, gently curved. Posterior margin rounded. Umbones 

 prominent, curved inwards and forwards. A narrow furrow, and a corresponding 

 internal rib, extend from the umbo to the ventral margin in a somewhat oblique 



dirt'ctiiin, and the furrow is crossed by small ril).s. A liruad and strong internal ril> 



31 



