108 CRETACEOUS LAMELLIBRANCHIA. 



1895. Thetis minor, F. Vo/jel. Holliuidisch. Kreide, p. 58. 

 l'JOO A. Wollemann. Die Biv. u. G-astrop. d. deutsch. u. holland. 



Neocoms, p. 118. 



Nou 1846. A. d'Orbigny; Pal. Franc. Terr. Oct., vol. iii, p. 453, pi. 



ccclxxxvii, figs. 4 — 7. 

 1850. d'Orbigny. Prodr. de Pal., vol. ii, p. 136. 



1868. — E. v. Eichwaltl. Lethaea Rossica, vol. ii, p. 707, pi. xxvi, 



fig. 6. 



Deseri})tio)i.— Shell oval, rounded, convex, slightly (sometimes moderately) 

 inequilateral ; length rather greater than height. Margins forming nearly 

 regular curves ; anterior margin less convex than the posterior, making a rounded 

 angle where it meets the nearly straight antero-dorsal margin. Umbones promi- 

 nent, rather broad, close together, more or less curved forward. Lunular region 

 depressed, not limited. Postero-dorsal region sometimes slightly compressed. 



Ornamentation consists of slightly-raised concentric lines at regular intervals 

 with less distinct lines between ; and of regular rows of radial pits, which on the 

 posterior part of the shell are replaced by rows of short spiny projections or 

 minute tubercles. 



A long, acutely angular rib extends from the level of the posterior adductor 

 to near the umbo. The front part of this rib is continued to near the anterior 

 adductor, but is less prominent than the angular part ; at first it curves veutrally, 

 and afterwards dorsally, the last part being somewhat angular. 



Measurements : 



(3) (4) (5) (6) (7) 



27 26-5 25 23 30 

 25 25 24 21-5 27 



(1 — 6) Crackers, Atlierfield. 

 (7 — 11) Internal casts, Lower Greeiisaud, Shanklin. 



Affinities. — Pictet and Campiche separated, but with considerable hesita- 

 tion, the examples of Thetironia found in the Crackers of Atlierfield from 

 those found in the Ferruginous Rock of Shanklin. The former they referred to 

 T. laevigata, d'Orbigny (icon Sowerby) ; whilst the latter are typical of T. minor. 

 There is, as stated by those authors, considerable difficulty in comparing 

 specimens from the two localities owing to their different states of preserva- 

 tion. Those from Atlierfield have the shell well preserved in nearly all 

 cases, although not uncommonly the original form has been somewhat modified 

 by crushing. The specimens from Shanklin, on the other hand, are nearly always 

 casts, but owing to the hardness of the rock they retain their original form 

 more perfectly. 



Pictet and Campiche thought that the Atlierfield form was less convex and 





(i) 



(2) 



Length 



31 



30 



Height 



28-5 



29 



(8) 



(9) 



(10) 



(ii) 



20 



20 



17-5 



1 7 mm. 



23-5 



18 



10-5 



1(3 „ 



