256 CRETACEOUS PELECYPODA 



margin is wholly toothed, which is not very usual in Lticina, and this, as likewise 

 the great thickness of the shell, rather partakes of the characters of Cof^bis ; but as 

 the anterior muscular scar is considerably more elongated than the posterior, and 

 extends within the space included by the pallia! impression, I consider the generic 

 determination as a Lucina to be the more correct one. 



Locality. — Ninnyoor, in white limestone; common. 



Formation. — Arrialoor group. 



2. Lucina fallax, Forbes, PI. XIII, Eigs. 13, 15-17 ; and PL XIV, Eigs. 3-5, 7-8. 



1846. Luc. fallax, Forbes, Trans., Geol. Soc, London, vii, p. 143, pi. xvii, fig. S—idem auctorum. 



i. testa orbiculata, nonnunquam paulo longiore quam. alta, cequilaterali, plus 

 minusve compressa, ad marginem interdum lamellose incrassata (vide tab. xiii, Jig. 13)^ 

 in superficie lamellis aciitis concentricis distantioribus et striis minutissimis inter- 

 positis ornata ; kmula parva, excavata ; area elongata, angustissima, profunda, liga- 

 mentum marginibus aciitatis fere omnino tegente ; cardine dentibus cardinalibus et 

 lateralibus sicut in Lucinis typicis instructo ; testce superficie interna minute scrobicu- 

 lata; impressione musculari antica august a, valde elongata atque flexuose curvata, 

 postica breviore ; margine interno Icevigato. 



PI. xiii, fig. 13. PI. xiii, fig. 16. PI. xiv, fig. 3. PI. xiv, fig. 5. PI. xiv, fig. 7. 



Height of shell : its length... 0'97 0*90 0*89 0-93 1-04 



Thickness „ : „ ... 0*61 0-40 0-42 0-46 0-50 



This is an extremely variable species. Its form is always more or less approach- 

 ing to orbicular with slightly prominent median beaks ; the lunula is small and 

 excavated, the areal margins sharp and almost touching each other above the liga- 

 ment ; the surface is covered with numerous sharp concentric lamellae, between 

 which fine striae* are situated ; sometimes the lamellae are especially numerous near 

 the margin, producing here a great thickness of the shell. The hinge-teeth, the 

 finely scrobiculate internal surface of the shell, muscular scars, and pallial impres- 

 sion do not differ in any marked way from those of other Lucince. 



With regard to form and thickness of the shell several varieties may be dis- 

 tinguished. 



The specimens figured on pi. xiv, figs. 3-5, may be regarded as typical ; the 

 shells are sub-orbicular, slightly longer than high, and the valves are very evenly 

 rounded, the thickness of the shell being more than f ths of its length. 



Pigs. 15-17 on pi. xiii represent a compressed variety, the shells are also some- 

 what longer than high, the valves are tumescent near the beaks, but much atten- 

 uated towards the margin ; the thickness of the shell is not more than fths of its 

 length. 



Pigs. 7-8 on pi. xiv represent a third variety, the shells are nearly orbicular, 

 with pointed beaks, with thick convex valves ; the thickness of the shell is equal to 

 one-half of its length. 



^ These are not sufficiently well expressed in figs. 3 and 5 of pi. xiv. 



