468 CRETACEOrS PELECYPODA 



I have not seen more than the figured lower valve of this remarkable species. 

 The radiating ribs all become obsolete near the margin and the pallial impression is 

 crenulated in its entire extent, which character readily distinguishes this species 

 from any others reported from cretaceous deposits. 



3. OsTREA [Alecthyonia] amoepha, Sowerby^ PL XLVIII, Eig. 3. 



1846. Ostrea amorplia, Sow., apud Eorbes, Trans. Geol. Soc, Lond., vii, p. 157, pi. 15, %. 24i~eadem 

 auctorum. 



Ostr, [Alectr,] testa irregulariter sub-rotundata, conveximcula, crassa, radiatim 

 incBqualiter cost at a : costis ad marginem plicis nonnultis concentricis intersectis, 



I have unfortunately very little to add to the meagre description of this 

 problematic species. The only specimen in our collection is very imperfect, but 

 it indicates a species allied to O. Desori, Ooq., (Monog. Ostr. cret., 1869, p. 117), 

 and may be identical with it. The geological position of both also corresponds, 

 Desori being from the Oarentonien. Eorbes' figure is based upon a very insuflS.- 

 cient fragment, exhibiting the interior of it ; it is the only one that I could find 

 amongst Eorbes' originals. 



Locality. — Near Serdamungalum, in bluish grey sandstone. 



Formation. — Trichinopoly group. 



4. OsTEEA [Alecthyonia] caeinata, Lamarck, PL XLVIII, Eig. 5 ; PL XLIX 



Eigs. 1—2. 



1810. Ostrea car inata,'L2Lm., Ann. d. Museum, viii, p. IQ^-.—eadem Coquand, Monog. Ostr. cret., 1869 

 p. 129, cum synon. 



O. \_Alectr.'\ testa elongata, angusta, curvata, mquivalm ; valvis costis numerosis 

 ad medium complicatis, et irregulariter dichotomis, sub-rugosis, in latere antico 

 concavo, simplicibus, in latere convexo abrupte descendentibus, supra ad marginem 

 elevatiusculis, nonnunquam duabus vel tribus seriebus tuberculorum coronatis ornatis • 

 auriculis parvis, aut sub-cequalibus, aut sub-obsoletis ; umbonibus scepissime paulo 

 incurvatis ; lig amentum in fovea ampla valvulce dextrce, Jugo valv, sinist, congruente 

 situm ; impressione muse, ovato elongata, prope marginem cardinalem posita ; margine 

 interior e valvarum undique acute dentato. 



Shell generally moderately curved, always very narrow, but often growing 

 to a very considerable thickness. The ribs are numerous and sharp, irregularly 

 dichotomous, somewhat more raised on the upper convex than on the concave 

 edge and occasionally ornamented with two or three rows of tubercles. The 

 margins of both valves are very strongly dentated. The beaks are depressed and 

 generally slightly twisted, the auricles being either developed of small size, or 

 nearly obsolete. On the anterior portion of the concave side the ribs are always 

 simple, slightly curved, very much resembling those of O. ungulata. The liga- 

 ment of the right valve is lodged in a deep groove which corresponds to a flattened 



