PTERTA. 65 



by a median step-like fold (fig. 3 a). A narrow part adjoining the posterior ear 

 slopes rather rapidly. 



Anterior ear of moderate size, convex, much higher than long. Posterior ear 

 large, united to the whole of the postero-dorsal margin of the valve; posterior 

 margin of ear slightly concave or sinuous, forming an obtuse angle with the hinge- 

 line and also with the postero- ventral margin. 



Ornamentation consists of numerous radial ribs which are straight or slightly 

 undulating, and extend over the larger part of the valve. On the posterior ear the 

 ribs are narrow and separated by broad, flat or slightly concave interspaces. On 

 the flattened part of the valve the ribs are rather more rounded and become less 

 distinct towards the postero-ventral margin in large specimens. In front of the 

 main ridge the ribs are closer together and the interspaces very narrow ; on the 

 anterior part of the valve and on the anterior ear, ribs are either absent or 

 indistinct. Numerous, close-set, regular, concentric linear ridges cross both ribs 

 and interspaces. 



In small specimens (figs. 4 a, b) having the ornamentation well preserved, the 

 ribs on the flattened part of the valve are narrow, rounded, distinctly limited, 

 and separated by broad interspaces ; new ribs are introduced in the middle of 

 some of the interspaces. The ribs and interspaces are crossed at regular intervals 

 by concentric ridges which form squares or oblongs with the ribs. On the posterior 

 ear similar ornamentation occurs, but the concentric ridges cut the ribs obliquely. 



Right valve not seen. 



Measurements : 



(1) (2) 



Umbo to postero-ventral extremity 80 . . 75 mm. 



Length of hinge-line . . . 49 . . 54 ,, 



(1, 2) Bkckdowu. 



Affinities. — The specimen from the Cenomanian of Le Mans figured by 

 d'Orbigny as Avicula anomala appears to be distinct from Sowerby's species on 

 account of its larger apical angle and its fewer, stronger, and more spiny ribs. See 

 also P. {Pseudoptera) haldonensis (below). 



The character of the hinge in this and the other species here included in the 

 sub-genus Pseudoptera is unknown ; consequently their systematic position cannot 

 be regarded as definitely determined. 



Remarks. — The only examples which I have seen are the type specimen, six 

 specimens in the British Museum, and two in the Museum of Practical Geology. 

 Those from Haldon have the ornamentation very perfectly preserved. 



Type. — In the Bristol Museum, from Blackdown. 



Distribution. — Upper Greensand (zone of ScMcenbachi<i rostrata) of Blackdown 

 and Haldon. 



9 



