Messrs. Davidson & Morris on some species of Erachiopoda. 251 



The general form of this Leptcena approaches that of Productus, 

 the species which it most closely resembles being the L. oblonga, 

 Pander. It has the same convexity and smoothness, and the 

 beak is similarly truncated by an apical opening ; the area and 

 pedicular opening has also some analogy to the Russian species, 

 but it differs in the contour of the dorsal valve, and the notch in 

 the front margin. (Bouchard MSS.) 



Leptcena Moorei, Davidson. PI. XVIII. fig. 1 a — e. 



Shell small, depressed, transversely quadrangular, ornamented 

 by numerous fine costse scarcely visible without a lens. Dorsal 

 valve slightly convex ; area double, as wide or wider than the 

 shell. Deltidium small, chiefly filled by the median tooth of the 

 ventral valve ; tooth with four depressions by which the muscular 

 fibres of attachment passed outwards. Length 1£ line, width 

 2 lines. 



The muscular impressions in the interior of both valves are 

 very peculiar to this species, and indicate that it did not attain 

 larger dimensions than those above assigned to it. 



This elegant small species was first discovered in the marlstone 

 beds near llminster by Charles Moore, Esq., to whom it is dedi- 

 cated ; and I trust ere long this gentleman will enrich science by 

 a detailed account of this interesting locality, which he has so 

 carefully investigated. The following section, forwarded by Mr. 

 Moore, shows the position of the bed containing the Leptcena : — 



1. Rubbly beds 6 to 10 feet with numerous Ammonites. 



2. Clay 8 inches. 



3. Yellow limestone 3 to 4 inches. 



4. Layers of clay 18 inches, Leptcena Pearcei. 



5. Leptaena bed 1 inch, Leptcena Moorei and L. Bouchardii. 



6. Marlstone 2| inches. 



7. Greenish sand 4 inches, containing numerous Belemnites. 



8. Marlstone. 



The discovery of four species of Leptcena in the lias is rather 

 an interesting fact, as proving that this genus, so abundant in the 

 palaeozoic period, existed also at the liassic epoch, although the 

 forms are considerably reduced in size from those of their pre- 

 cursors. The existence of Leptcena at the early part of the second- 

 ary period teaches also how cautious we should be in assigning 

 to any genus a limited duration in time, or that the lines of de- 

 marcation between successive periods are not so arbitrary regard- 

 ing certain typical forms as our first investigations would lead 

 us to infer. 



Leptcena Bouchardii, Davidson. PI. XVIII. fig. 3 a, b, c. 

 Shell very small, almost a perfect oval, surface smooth ; dorsal 

 valve very convex, ventral valve very concave, leaving only a small 



18* 



