PTERTA. 65 



by a median step-like fold (fi,n-. '^ (i). A iian'ow ])art adjoining the posterior ear 

 slopes rather rapidly. 



Anterior ear of moderate size, convex, much higher than lung. 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 tlie postero-ventral margin. 



Ornamentation consists of numerous radial ril)s whieli are straight or slightly 

 nndidating, 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 nari'ow ; on the 

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

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

 and interspaces. 



In small specimens (figs. 4ii, h) having the ornamentation well preserved, the 

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

 and separated by I)road 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, )iut the concentric ridges cut the ribs obliquely. 



Right valve not seen. 



MeasurcinriitK : 



(1) (2) 



ITml)0 to postero-ventral extremity 80 . . 75 mm. * 



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



(1, 2) Blackdowu. 



Affinities. — The specimen from the Cenonianian of Le Ahms figured l)y 

 d'Orbigny as Arinihi anoviuda appears to be distinct from Sowerby's species on 

 account of its larger apical angle and its fewer, stronger, and moi-e spiny ribs. See 

 also 1\ {Pi^endoptpm) haldovensis (below). 



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

 sub-genus I'seudoptera is unknown ; consequently their systematic position cannot 

 be regarded as definitely determined. 



BpiiKirh.^The oidy examples Avhich 1 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. 



Ti/pe. — In the Bristol IMuseuui, from Blackdown. 



Disfnbntiuii. — Upper Greensand (zone of ScJda'ubachia rostrata) of Blackdown 



and Haldon. 



9 



