88 CARBONIFEROUS LAMELLIBRANCHIATA. 



History), and I have permission to reproduce it here, PI. VI, fig. 4. Other and 

 better specimens, however, are in the same collection. 



The details of the species have been worked out from a fine and numerous 

 series from Settle in the Burrow Collection of the Woodwardian Museum, one or 

 two specimens showing important details of the interior; but, on the whole, 

 these shells are more gibbose than those of the same species from other localities. 



Although none of the Settle specimens have both valves preserved, I have 

 been fortunate enough to obtain perfect examples from other localities. The 

 curious subspiral form in single valves has a close resemblance to certain 

 Gasteropod shells. 



There appear to be two forms or varieties of this species, but whether or no 

 they are distinct enough to have different names I have been unable to decide. 

 One form, represented by figs. 20, 21, and 22, PL V, is flatter, more compressed, 

 and expanded. The other, figs. 1, 2, and 3, PL VI, are more convex, and have 

 the posterior superior angle less marked. Intermediate forms occur which 

 connect the two. This difference in concavity is not due to one valve being 

 more tumid than the other, for fortunately both convex and flat specimens of 

 both valves appear in the series. The flat valves resemble somewhat Myalina 

 amcena and M. ampliata of de Koninck, but the casts show the absence of any 

 rostral plates or strias on the hinge. 



The figures of Mytilus ampliatus, M. Wesemselianus , and M. Omaliusianus 

 given by de Ryckholt are extremely like those of the species under description ; 

 but de Koninck does not recognise them in his great work. I have not therefore 

 placed them as synonyms of this shell for the present, but have merely indicated 

 these forms for comparison. Should it be shown that the British and Belgian 

 species are identical, de Ryckholt's name " M. ampliatus " should probably stand 

 for the species, though this form is stated to come from the coal-field of Mons. 

 De Ryckholt's figures, however, are not to be trusted as conveying an accurate 

 idea of the shell which he intended to depict. 



Posidoniella. elongata, Phillips, sp., 1836. Plate V, figs. 15 — 18 a, 19, 19 a; 



Plate VI, fig. 6. 



Modiola elongata, Phillips, 1836. Geology Yorkshire, pt. 2, p. 210, pi. v, 



fig. 24. 

 — — Morris, 1854. Cat. Brit. Toss., p. 209. 



? Myalina lamellosa, pars, de Koninck, 1842. Anim. Toss. Beige, pi. iii, fig. 6 c. 

 elongata, JBroun, 1849. Ulust. Foss. Conch., p. 174, pi. lxxii, fig. 43. 

 Mcidiola elongata, Etheridge, 1885. Brit. Foss., vol. i, Palseoz., p. 285. 



