LAMELLIBRANCHIATA. 129 



Affinities and differences. Characterised by its rhomboidal 

 form. In Britain we have only M. lima, Defr. to distinguish it 

 from, a species to which it bears considerable resemblance ; but the 

 front margin in M. lima is truncated, whilst in M. rhomboidalis it 

 is obtusely pointed ; in our new species the beak is less incurved, 

 the hinge line is, as a rule, more curved, and the dorsal valve is 

 more swollen than in M. lima ; also this latter species is ornamented 

 with "small granulations or short, hollowed spines" (p. 43, Davidson), 

 which I have not found in M. rhomboidalis. Again, the lines of 

 growth are more marked. 



In geological times the two species seem always to have been 

 separated, M. lima ranging only from the Upper Gault to the 

 Chalk. 



Locality. Brickhill. 



Waldheimia Bonneyi, sp. nov. 

 (Plate VII. fig. 4, a, b, c) 



Shell thick, oval-pentagonal, usually inflated; pointed behind, 

 truncated and straight or sinuated in front. It is broadest near 

 the centre, and tapers behind to the beak. The surface is marked 

 with interrupting lines of growth of varying intensity, valves about 

 equally convex, deepest near the centre. 



Ventral valve moderately convex, impressed towards the front 

 so as to form the lateral ridges, and gently carinated in the pos- 

 terior region. Beak rather prominent, slightly incurved, beak 

 ridges sharp. 



The hinge area is large and flattened. Deltidium large, in 

 two pieces. Foramen small. 



The Dorsal valve is pentagonal, somewhat inflated, the sides 

 gently curved or with a slight angle. Front sinuated. The looj) 

 is long and reflected. 



This species varies considerably in its form much in the same 

 way as does its near ally W. psetidojurensis. Some of the specimens 

 are flattened so as to pass into that species by regular gradation, 

 but the typical ones are always more inflated. The less truncated 

 forms approach W. Wanklyni. Some specimens are much 

 thickened on the front margin, some are broadly truncated in front, 

 and others are almost pointed. 



K. 9 



