176 DEVONIAN FAUNA. 



Remarks. — There is some difficulty in defining the specific limits of this 

 abundant shell on account of its variability, and the likeness of some of its shapes 

 to species of Productus. 



Its typical form has nearly circular margins and fairly numerous small spinous 

 markings ; it is well represented by Murchison's original figure, and by those of 

 Sowerby's Leptsena caper ata. Specimens which are rather more transverse than 

 this, and have rather more numerous spinous markings, occur commonly at Pilton 

 (PI. XXI, fig. 10). In another variety the thorn-like spinous markings are replaced 

 by strong and more or less continuous ribs, and the transverse wrinkles are 

 confined to the posterior half (PI. XXI, fig. 8) ; these shells seem in their usual 

 imperfect condition sometimes so similar to P. scabricidus that it is not easy to 

 draw the line between them, but they appear distinguishable by their smaller 

 umbones, their broad hinge-lines, and their long hinge-spines. Again, on the other 

 side of the typical form we find one of the commonest variations presented by 

 large elongate flattish shells, in which the umbones are very small, and the spinous 

 markings are minute, short, multitudinous and quincuncially arranged (PI. XXI, 

 figs. 7, 9) ; in these the transverse wrinkles are very numerous and prominent, and 

 sometimes produce with the spinous markings a zigzag appearance. At first sight this 

 variety has a distinct appearance, but it is most probably only an aged or extreme 

 form of the present shell. Lastly, in a few small shells we find the spinous mark- 

 ings very few, acicular, and confined to the more central parts of the shell, and the 

 transverse ridges prominent and straight (PI. XXI, fig. 11) ; these may be regarded 

 as immature shells. While, however, the above variations may be noted, it must 

 be added that they are not constant, as they are united by intermediate forms, and 

 few shells are exactly alike. There seems, therefore, every reason to believe that 

 they all belong to a single species. 



The strong erect spines of the hinge of both valves sometimes reach a length 

 of more than 10 mm. (PI. XXI, fig. 9). Specimens occasionally occur in which 

 the body-spines are seen to be elongate, crowded, and hair-like over the whole 

 surface. A decayed specimen from Baggy retains them wherever the shell is not 

 obliterated, both on the shoulders and the front portion of the valve (PI. XXI, 

 fig. 6). While the extreme elongation in shape of some specimens is largely due 

 to contortion, it is probably not to be wholly accounted for by this cause. 



Two other variations not occurring in the Barnstaple area must be noticed. 

 At South Petherwyn the shells called by Phillips Leptsena membranacea, 1 but 

 united by Davidson to this species, are common. These are small and flat ; 

 their spinous markings are very few and blunt, while their wrinkles are numerous, 

 prominent, and zigzagged. All the specimens I have noticed are dorsal valves, 

 but a ventral valve is figured by Davidson. Again, in the same beds occur equally 

 1 1841, Phillips, ' Pal. Fobs.,' p. GO, pi. xxv, fig. 101. 



