342 DR WHEELTON HIND ON THE LAMELLIBRANCH AND 



finely radially striate near its lower margin in the more recent part of the valves. 

 Shell very thin. 



Dimensions. — Specimen No. T4503 B . a right valve, measures : antero-posteriorly, 

 10 mm. ; dorso-ventrally, 10 mm. PI. I. fig. 5. 



Locality. — Midlothian, Bilston Burn near Pol ton, bed 27 to 30 feet above the 

 Castlecary Limestone. 



Observations. — Several specimens of the right valve of this species, but only two of 

 the left, have been obtained. Comparing the specimens with the figures given by Meek 

 in the report on Nebraska (op. supra cit), it will be seen that he gives a very perfect 

 figure of the right valve, with which our specimens agree ; but it would seem that he 

 copied Geinitz's figure of the left valve, which I have reasons for thinking may have 

 been a right valve with the ears imperfectly exposed, because the left-hand ear in 

 that specimen has a falcate margin and should therefore be the posterior. Amongst 

 the specimens from the Bilston Burn is a left valve (fig. 4, PI. I.) which shows in its 

 umbonal region a surface free from radiating ribs, but towards the margin ribs gradually 

 arise and become close and numerous. The specimen is a left valve, and I have sus- 

 pected that it is the left valve of the smooth right valves which agree with the 

 American right valve in every essential character. 



In vol. v. of the Geological Survey report on Illinois, Worthen gives the figure 

 of the hinge plate, showing a number of parallel cartilage pits, closer on the anterior 

 than on the posterior side, and a central cartilage cavity. If he is correct, the shell 

 cannot be referred to Aviculoyecten, but will require a new genus to be created for it. 



Meek figures a left valve with radial ribs, from Bed C, Nebraska, in which 

 ? A. neglectus occurs, as A. coxanus, M. and W. It is possible that this may be the 

 left valve, and that I am dealing with partially decorticated specimens, and am wrong 

 in thinking that the valve was not radially ribbed in the umbonal region. 



Posidoniella Isevis, Brown, sp., 1841. (PI. I. fig. 15.) 



For synonymy, vide Hind, Pal. Soc, 1897, Brit. Garb. Lamell., p. 94. 



I have referred certain small shells to this species, which is extremely common in 

 the shales of the Millstone Grit in the Midlands of England. The specimens are not 

 good, and are small and crushed, but on the whole fairly distinctive of the species; it 

 seems to have been very rare in Scotch localities, and dwarfed. 



Myalina Verneuillii, M'Coy, sp., 1854. (PI. I. fig. 16.) 



For synonymy, vide Hind, Pal. Soc, 1897, Brit. Garb. Lamell., p. 115. 



I have referred some fragments of a large shell with a broad alate posterior end to 

 this species, but they are also very suggestive of M. redesdalensis. Some smaller 

 specimens one can positively refer to M'Coy's species, which has been obtained in beds 

 of the Millstone Grit series near Marsden, on Pule Hill, Yorkshire. M. Fleminyi occurs 



