58 CRETACEOUS LAMELLTBRAXCHIA. 



1889. AvicuLA iN.EQUiVALVis, G. W. LamiiliKjh. Quart. Journ.Geol. Soc.vol. xlv, 



p. 615. 

 1895. — CoRNUELiANA, G. Maag. Zeitschr. der deutsch. geol. Gesellsch., vol. 



xlvii, p. 2G7. 



1895. — — F. Vofjel. Holliiudisch. Kreide, p. 55. 



1896. — — A. WoUemuini. Zeitschr. d. deutsch. j,'eol. Gesellsch., 



vol. xlviii, p. 842. 



1900. — — — Die Biv. ii. Gastrop. d. deutsch. u. 



hoUiiud. Neoeoms (Ahhandl. d. k. 

 preussisch. geol. Land., N. P., pt. 

 31), p. 52. 



1901. OxYTOMA iN.EQuivALVE cdf. MACROPTERA, L. Waagc'ii. .Talirli. d. k.-k. geol. 



Reiohsaust., vol. li, pp. 12, 

 15, pi. i, figs. 7, 14, 15. 



Description. — Shell obliquely oval, roundtHl. Hoiglit a little greater than length. 



Left valve moderately convex. Anterior ear triangular. Posterior ear larger 

 and longer than the anterior. Surface of valve with from 12 to 21 main rib.s which 

 are rounded, and form projections on the margin of the valve. Between the main 

 ribs are broad flat interspaces in the middle of each of which a smaller rib occurs, 

 and between these secondary ribs and the main ribs one or more still smaller ribs 

 are found. On the middle and posterior parts of the valve the ribs are nearly 

 straight, Ijut on the anterior part they curve forward. Similar ribs occur on the 

 anterior ear; on the postei'ior ear nuich smaller ribs are pi-esent, and growth 

 lines are seen. Fine concentric ridges cross both i-il)s and interspaces. 



Right valve nearly flat, with many small, sometimes irregular ribs, which may 

 be alternately large and small. Anterior ear ratlier small, witli a well-marked 

 byssal sinus. Posterior ear large, pointed, with small radial riljs. 



Medsurements : 



(1, 2) Speeton Clay (D 1), Speetou. 

 (3) Claxhy Ironstone, Claxby. 



Affinities. — P. {O.rijtoma) CurniwUdiin is distiuguisheil from tlie other Cretaceous 

 species of Oj'ijtoina l)y the broad interspaces on which several smaller ribs occur. 

 It belongs to the persistent and variable series of forms, ranging from the Rh;etic 

 to the Chalk, of which Ptcria iniequifulvis (Sowerby) is the type, and it is regarded 

 by L. Waagen as only a variety of that species. 



In most of the English specimens the main ribs are more numerous but less 

 prominent than in the examples figured by d'Orl)igny and by I'ictet and Campiche. 



