404 



UnionidcB of the Country of the Iguanodo7i. 



half being nearly one fifth longer than the anterior, and com- 

 pressed above along the margin of the ligamental space. The 

 substance of the shell is very thick and strong, and marked 

 externally with concentric longitudinal striae or grooves. The 

 summit is rounded, rather antero-dorsal, slightly inclined for- 

 wards, and the beaks or umbones decorticated, as in most of the 

 recent shells of the genus. The base or circumference is entire ; 

 and the margin or internal lip smooth and thick. The ligament 

 is external and post-apicial ; its horny character is still preserved, 

 and its surface is marked with transverse rugas, as in a recent 

 state ! The inner surface of the shells is smooth, with the ex- 

 ception of a few irregularities produced by the nacreous deposit 

 being of unequal thickness. The structure of the hinge and 

 the number and situation of the muscular impressions do not 

 materially differ from those of several recent species ; in the rel- 

 ative situation of the two cardinal teeth and the lateral tooth- 

 plates, the fossil approaches nearer to the American U?iio purpu- 

 riatus* than any recent species with which I have been able to 

 compare it. 



Unio Valdensis, (G. A. M.) 

 Wealden formation, Isle of Wight. 



Fig. 1. View of the left valve, and part of the right seen above : closed. — (Re- 

 duced sketch ; for dimensions, see bottom of p. 403.) 



w, ID. Decorticated umbones. x. Ligament. 



* So named in the British Museum collection. 



