48 Proceedings of the Royal Physical Society. 



is very common at the typical locality, Cults, Fifeshire, where 

 it may be gathered in large quantities. Formerly, Myalina 

 was referred to the Mytilidie, but Messrs Meek and Hayden 

 have shown * the shell structure of American species to be 

 prismatic, like that of the Aviculidae and Dreissenidae. This 

 character added to the undoubted inequality of the valves 

 and broad cardinal area, places the genus, they think, rather 

 in the Aviculidse than the Mytilidse. 



In the Uppermost Devonian and some portions of the 

 Carboniferous the genus Posicloiiomya occurs of large size, 

 but in the Carboniferous series of Scotland the species are 

 small. 



P. corrugata is one of those shells which attracts attention 

 from the collector on account of its highly gregarious nature. 

 It literally occurs in thousands at certain horizons, especially 

 that of the Calderwood Cement-Stone in the Lower Limestone 

 group of E. Kilbride.f 



Pinna and its allies are now regarded as a separate family, 

 and must in future be so considered. Very little appears to 

 be known about the internal characters of the Carboniferous 

 species referred to Pinna, but there certainly appears to be 

 very little difference between P, flahelliformis (Martin), tlie 

 common species, and those now in existence. Three or four 

 species occur in Scotland, and in places are common; 

 for instance, at Bogie quarry, near Kirkcaldy. At Bishop- 

 briggs, in the representative of the Orchard Cement-Stone, 

 Messrs Armstrong and Young have observed specimens of 

 Pinna placed in an erect position, at right angles to the 

 strata.]: 



The Mytilidse is represented by shells referred to the 

 genus Modiola, and by the brackish water forms Ayithra- 

 coptera and Anthracomya. One shell also has been 

 placed in the genus Mytilns, and although not prepared 

 to deny the presence of this genus in Pala30zoic rocks, 

 I yet believe the species usually called by this name 



* Pal. Up. Missouri, p. 31. 



t For detailed descriptions see Mem. Geol. Survey, Expl. 23, 1873, pp. 71 

 and 103 ; Geol. Mag., 1874, i., p. 301. 



4: Trans. Geol. See, Glasgow, 1877, v., pt. 2, p. 252. 



