24 CRETACEOUS LAMELLIBRANCHIA. 



Lima (Plagiostoma) Mabkotiana, d'Orbigny, 1847. Plate V, figs. 6a, b, la, b. 



1847. Lima Marrotiana, A. d'Orbigny. Pal. Franc. Terr. Oct., vol. iii, p. 561, 



pi. ccccxxiv, figs. 1 — 4. 

 1850. d'Orbigny. Prodr. de Pal., vol. ii, p. 247. 



1889. — (Radula) Marrotiana, 0. Griepenkerl. Seuon. von Konigslutter 



(Palaeont. Abhandl., vol. iv), p. 39. 



Non 1850. -- Marottiana, A. Alth. G-eogn.-pal. Beschreib. v. Lemberg (Haidinger's 



Naturwiss. Abhandl., vol. iii, pt. 2), p. 240, 

 pi. xii, fig. 25. (L. AUJd, Favre). 



Description. — Shell of moderate convexity, oval, more or less trigonal. Antero- 

 dorsal margin rather long, nearly straight ; postero-dorsal margin much shorter ; 

 the remainder forming a regular curve. Umbones rather small, pointed. Apical 

 angle about 105°. Anterior area large, deeply depressed, sharply limited, 

 ornamented with 10 to 12 strong, rather narrow ribs, which bear, in places, small 

 nodular projections. Ears rather large, with growth-ridges, without ribs ; the 

 posterior rather larger than the anterior ear. 



Ornamentation consists of 30 to 32 broad ribs with a few smaller ribs near the 

 posterior margin ; the ribs are smooth, with flattened or somewhat rounded 

 summits, and are separated by narrow rounded grooves. The grooves show, in 

 places, transverse ridges and grooves which may extend to the sides of the ribs, 

 giving them a notched appearance. Near the ventral margin, especially in old 

 specimens, the ribs become more flattened and the grooves shallower. 



Measurements : 



(i) (2) 



Length . . . . 59 . 27 mm. 



Height . . . . 60 . 28 „ 



(1, 2) Upper Chalk (B. mucronata zone), Norwich. The measurements of the larger specimen are 

 approximate only. 



Affinities. — This is distinguished from other species found in the Chalk by its 

 strong ribs. In Lima Altld, Favre, the ribs are more numerous and not so broad. 



Lima Marrotiana differs from most of the species which are referred to 

 Plagiostoma in having much stronger ribs, but in other respects it agrees closely 

 with that sub-genus. 



Remarks. — The only specimens I have seen are from Norwich, where it appears 

 to be rare. The shell is usually more or less crushed, so that its proper outline is 

 distorted. 



