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SUPPLEMENT TO THE BRITISH 



half their length from the beaks. Dorsal valve biplicated near the front, with a deep 

 rounded sinus between the plications. Ventral valve with a projecting central rounded rib, 

 commencing to rise at about half the length of the valve from the beak and extending to 

 the front, with a depression on either side. Beak'very short, incurved, and truncated by a 

 large circular foramen, slightly separated from the hinge-line by a narrow deltidium in one 

 piece ; beak-ridges obsolete. Surface smooth, marked at intervals by concentric lines of 

 growth ; frontal margin more or less deeply biplicated. Loop short, a little wider than 

 in T. expansa. Shell-structure with few and small perforations. Average length 16, width 

 10, depth 9 lines. 



Obs. Mr. Walker has devoted much care in his study of this species, which he 

 considers to be at once distinguishable from T. prcelonga by its peculiar shell-structure, 

 and in that it is more compressed at the sides, the foramen more approximated to the 

 hinge-line, and the beak shorter. He adds likewise that it is well distinguished from T. 

 biplicata by its shell- structure and general shape. The forms in which the plications are 

 less conspicuous approach most nearly in appearance to T. extensa ; but it differs from 

 that species in the shell-structure, in being wider, and in the ventral valve being flattened 

 towards the front margin. We are also informed that he had originally placed some 

 of the specimens of this species among the varieties of T. prcelonga and others among 

 those of T. extensa; but that Mr. A. Wanklyn having made him some microscopic 

 preparations of the shells of the Terebratula occurring at Upware, they had both 

 found that those doubtful specimens presented the peculiar accordant structure, quite 

 distinct from both T. extensa and T. prcelonga. 



Position and Locality. T. microtrema is abundant in the Lower Greensand at 

 Upware. Mr. Meyer has a specimen from Shanklin, Isle of Wight, a single valve from 

 Faringdon, and a doubtful specimen from Godalming. 



43. Terebratula Lankesteri, Walker. Sup., PI. Ill, figs. 9 — 11. 



Terebratula Lankesteri, Walker. Geol. Mag., vol. v, p. 402, pi. xix, figs. 2, 3, 1868. 



Spec. Char. Shell ovate, longer than wide, broadest about the middle, greatest 

 depth near the front or anterior half of the valves ; tapering to the beak, rounded or 

 slightly indented in front. Dorsal valve evenly convex and abruptly bent (at almost right 

 angles) near the front and lateral margin. Ventral valve very convex and gibbous, much 

 bent before reaching the front and lateral margin ; beak moderately incurved and 

 truncated by an oval foramen, separated from the hinge-line by a smallish deltidium 

 in one piece ; beak-ridges obsolete. Surface of both valves closely marked by 

 numerous fine, radiating, flexuous striae, which become more apparent, and assume a 

 peculiar zigzag appearance as they near the margin, occasioned mainly by these last 



