168 CRETACEOUS LAMELLIIIP.ANCHIA. 



18y5. Thetis minok, F. Vorjel. Holliiudisch. Kreide, p. 58. 



I'JOO — — A. WoUemann. Die Biv. u. Gastrop. d. deutscb. u. bulliiud. 



Neocoms, p. 118. 



Nou 1846. — — A. d'Orhi(jiiij. Pal. Frau?. Terr. Crct., vol. iii, p. 4oo, pi. 



ceclxxxvii, fi^'s. 4 — 7. 



— 1850. — — d'Orh'ujiuj. Prodi-, de Pal., vol. ii, p. 136. 



— 1868. — — E. V. Eickwald. Letliaea Rossica, vol. ii, p. 707, pi. xxvi, 



fig. 6. 



iJescrijjtiuii. — Shell oval, ruiinded, convex, slightly (sonietimos moderately) 

 inequilatei-al ; length rather greater than height. Margins forming nearly 

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

 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, whicli 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 ciu'ves ventrally, 

 and afterwards dorsally, the last part being somewhat angular. 



Measurements .- 



(3) (4) (5) (0) (7) (S) (9) (10) (11) 



27 20-5 25 23 30 20 20 17-5 17 iiim. 

 25 25 24 21-5 27 23-5 18 i&b 16 „ 



(1 — C) Crackers, Atlierfield. 

 (7 — 11) lutei'ual casts, Lower Greeusand, Shaukliu. 



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

 tion, the examples of Tketiroiiia found in the Crackers of Atliei'field from 

 those found in the Ferruginous Rock of Shanklin. The former they referi'ed to 

 T. laevigata, d'Orbigny (uo)i Sowerby) ; whilst the latter are tyi)ical of T. miiuir. 

 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 Atherfield have the sliell well presei'ved in nearly all 

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

 by crushing. The specimens from Shaukliu, 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 Atherfield form was less convex and 



