74 DEVONIAN FAUNA. 



In P. amabilis, Barr.,^ the umbo is central and direct, and more acute. 



In P. intricans, Barr.,^ the threading is complicated by every fourth thread 

 being larger. 



In P. ohsequens^ Barr.,^ the threading is somewhat similarly complicated and 

 the umbo more central. 



In P. expansa, Barr./ the ribs, though expanding laterally, become small again 

 almost immediately. 



P. gracilis, Barr.,^ closely resembles the specimen in the Barnstaple Athenaeum 

 in shape, and apparently is rather coarsely threaded, but it shows no signs of the 

 central diminution of ribs. It is probable, however, that our shell has not retained 

 its true shape, and the Bohemian shell too has signs of being distorted. 



Cardium rigidum, F. A. Romer,^ appears congeneric, but seems distinguished 

 by having a more anterior umbo, rather more numerous ribs, no transverse 

 threading, and no central diminution of the ribs. 



P. grandiSy Whiteaves,'^ is much larger and more transverse, and has still 

 wider lateral ribs, and no transverse threads, 



P. potens, Hall,^ comes very close, but seems to have a more central and erect 

 umbo and less strong concentric marks, and its ribs are fainter above. 



II. Family — Grammysiid^, Rudolf Hoernes, 1884. 

 1. Genus — Leptodomus, IPCoy, 1844. 



Shell oblong, transverse, trapezoidal, convex ; anterior end rounded ; posterior 

 end truncated and gaping ; surface transversely ribbed, generally with a central 

 constriction ; umbo arched, prominent, anterior ; lunule deep ; hinge straight, 

 toothless ; muscular impressions slight. 



It appears to bear strong resemblance to the genus Pholadomya, but its pallial 

 line is stated to be entire. 



1 1881, Barrande, ' Syst. Sil. Bohem.,' vol. vi, pi. cxx, 6gs. 6, 7, and pi. cccxxxvi, figs. 9 — 11, Et. G. 



2 Ibid., pi. cxxxv, figs. 1—7, Et. E. 



3 Ibid., pi. cli, figs. 5—18, Et. E. 



4 Ibid., pi. cliii, figs. 7—9, and pi. cclxxxiv, figs. 24, 25, Et. F. 



5 Ibid., pi. cccxi, figs. 7 — 9 ; pi. cccxxv, figs. 8, 9 ; and pi. cccxxxiii, figs. 1 — 6, Et. G. 



6 1866, F. A. Eomer, 'Beitr. Harzgeb.,' pt. v, p. 10, pi. xxxv, fig. 1. 



7 1891, Whiteaves, ' Cauad. Eecord of Science,' Ann. 1891, p. 401, pi. i. 



8 1885, Hall, ' Pal. N. T.,' vol. v, pt. 1, No. 2, p. 422, pi. Ixix, figs. 8, 10. 



