150 CRETACEOUS LAMELLIBRANCHIA. 



1873. Cyfrina trapezoidalis, H. B. Geinitz. Das Elbthalgeb. in Sacliseu 



(Palaeontographica, vol. xx, 

 pt. 1), p. 229, pi. 1, fig. G 

 (?5). 



1889. Cypricardia E. Holzapfel. Die Mollusk. Aachen. Kreide 



(Palaeoutograpliica, vol. 

 xxxv), p. 179. 



1897. Trapezium trapezoidale, H. Woods. Quart. Journ. Geol. Soc, vol. liii, p. 



391, pi. xxviii, figs. 9, 10. 



1901. Cypricardia trapezoidalis, F. Sturm. Jalirb. d. k. preussisch. geol. 



Landesanst. f iir 1900, vol. xxi, 

 p. 80, pi. vii, fig. 5. 



1902. A. Wollemann. LiineburL?. Kreide (Ablmndl. 



d. k. preussisch. geol. 

 Landesaust. N. F., Heft. 

 37), p. 78, pi. ii, fig. 3; 

 pi. iii, fig. 1. 



Description. — Shell trapezoidal, rounded, inflated. Ventral margin slightly 

 curved, roughly parallel to the dorsal margin, and passing gradually into the 

 rounded anterior margin. Posterior margin oblique, forming an obtuse angle with 

 the dorsal margin and an acute angle with the ventral margin. Unibones near the 

 anterior end, much curved. A sharp, gently curving carina extends from the 

 umbo to the posterior angle and cuts off a triangular and slightly concave area. 



Ornamentation consists of fine concentric lines. 



Measurements : 









(i) 







(2) 



(3) 



Length 







25 







23 



21 mm. 



Height 







17 







14 



14 „ 



Thickness 



. 





10 







15 



15 „ 





(1- 



-3) 



Chalk 



Rock, 



Ci 



ickhamsley. 





Affinities. — Wollemann considers that Trapezium galicianum (Favre) 1 is not 

 distinct from this species, but is founded on an older example than the type of 

 T. trapezoidale. According to Wollemann, various changes in the form of the 

 shell take place during growth. Thus, in the older specimens the valves become 

 more convex, the length relatively less, the posterior margin less oblique, the 

 ventral margin more curved, the outline less trapezoidal, and the unibones less 

 curved. This view is probably correct, but none of the English examples which I 

 have seen pass beyond the stage represented by Romer's type. 



In Trapezium tricarinatum (Homer)' 2 the unibones are less anterior in position 

 than in T. trapezoidale. 



1 ' Moll. Foss. de la Craie de Lemberg ' (1869), p. 109, pi. xii, fig. 3. 



- 'Die Verstein. d. nord-deutsch. Kreidegeb.' (1841), p. 74, pi. ix, fig. 23. 



