68 CRETACEOUS LAMELLTBRAXCHIA. 



? 1841. AvicuLA ccERULESCENs, F. A. Bottler. Die Verstein. d. iiurd-deutsch. Kreide- 



geb., p. 64. 

 1850. — —A. (VOrhiyii!/. Prodr. .le Pal, vol. ii, p. 249. 



1888. — cjenvLESCEms A. Pi'i-iin. Hist. Terr, de Craie, p. 155, pi. i, fig. 14. 



? 1889. — cffiKXJLESCENs E. Holzap/il. Dip Mollusk. Aachen. Kreide (Palae- 



oiitographica, vol. xxxv), p. 227. 

 1897. — — A. Heiuiig. Kevis. Lamell. i Nilssou's 'Petrif. 



Siieeana,' p. 54, pi. iii, figs. 25 — 27. 



Descripfloii. — Shell rather small, ol)lii|ue, triangular. Anterior margin slightly 

 convex oi' nearly straight. Umbo rather near the anterior e.xtreinity, sometimes 

 curved slightly backwards. Anterior ear small, not distinctly marked off from the 

 rest of the valve. Posterior ear large, triangular, its inner margin not limited, its 

 posterior margin slightly concave and continuous with tlie postero- ventral margin 

 of the valve. Median part of the valve raised, extending obliquely backwards, some- 

 times subcarinate anteriorly. In front of this raised part the valve is bent more or 

 less sharply; behind, it is compressed gradually. Surface with weak radial ribs, which 

 are straight or slightly undulating, and bear small spiny or scaly projections some- 

 times close together, sometimes more or less widely separated. The ribs may 

 occur on the anterior part only, or may be present over the entire shell, including 

 the ears. Often on the anterior part they are closer together than elsewhere. The 

 number of ribs and the width of the flat interspaces vary in different specimens. 

 New ribs may be introduced in the interspaces at varying distances from tlie umbo. 

 In some specimens numerous fine concentric lines are seen. 



Medsaremeuts : 



(1) (2) (3) 



Length . . 11 . 11 , 10 nnii. 



Height . . 13 . 12 . lO-.-) „ 



(1) A. qiiadratus zone. East Hariibain. 



(2) B. miicronata zone, Norwich. 



(3) „ „ Clarendon. 



Apiiiifics. — The imperfect specimen from the Lower Senoniau of Brunswick 

 figured by (i. Midler' as Avlcnla sp. may perhaps be an example of P. rirntlescens. 



Avicnla 'jlahru, Reuss," resembles in t'niui /'. c(Cridesceus, but is distinguished 

 by the absence of radial ril)s. 



Avicnla subnodosa, Hagenow,'' from tlio Senonian of Riigen, is pcrliaps iden- 

 tical with P. cmrulescens, but in tlic absence of a figure of tlie former 1 am unable 

 to make a comparison. 



' ' Mollusk. d. Uuterscn. v. Brauusclnvoig u. Ilsedy ' (ls;»8), p. oi*, pi. v, fig. 10. 

 2 ' Die Verstein. der bohm. Kreidefonnat.' (1846), pt. 2, p. 22, pi. xx.\ii, figs. 4, 5. 

 ■■* 'Neues Jahrl), fiir Min.,' etc. (1842), p. 559. 



