148 CARBONIFEROUS LAMELLIBRANCHIATA. 



lower, nearly straight. The umbones arc small, slightly raised, situated a little 

 anterior to the centre of the hinge-line. The posterior end of the shell is not so 

 deep from above downwards as the anterior. Shell thin. 



Interior. — Unknown . 



Exterior. — The surface is ornamented with numerous, somewhat irregular, con- 

 centric strias and. rugas of growth. The microscope shows the surface of the shell 

 to be covered with numerous small pittings. 



Dimensions. — PI. XXV, fig. 12, measures— 



Antero-posteriorly . . . .34 mm. 



Dorso-ventrally . . . .23 mm. 



Localities. — In shale a few feet under the Arden Limestone, Thornliebauk, 

 Renfrewshire, and Linn Spout, Dairy, Ayrshire. 



Observations. — All the shells of this species that I have seen are crushed and 

 flattened. They are very numerous and occur in all stages of growth. The fossils 

 are in calcite covered by a deposit of iron pyrites, a condition which is of important 

 aid in showing that they belong to the Mollusca rather than to the Crustacea. 



The generic affinity is doubtful in the absence of details of the interior, but the 

 shells have none of the special characters of Posidonomya, and I am quite at a loss 

 to account for their reference to this very distinct and characteristic genus. The 

 shells are not oblique, and have no posterior wing-like projection. For the present 

 I place the species in the genus Edmondia, because there is no lunule and no 

 escutcheon, and the hinge-line appears to be simple and erect. Whatever ligament 

 there was seems to have been internal. 



Prof. T. Rupert Jones has lately described some bivalve shells from Russian 

 Carboniferous beds as Posidonomya subovata (' Geol. Mag.', dec. iv, vol. viii, p. 434, 

 pi. xvi, figs. 8 — 15). Judging from the figures, they are exceedingly like E. punc- 

 tatella, only much smaller ; and, except that the punctate ornament appears to be 

 absent, they could not be satisfactorily separated from that shell. Whether 

 identical or not, the Russian shell should be removed from the genus Posidonomya. 



An examination of the very numerous specimens in the collection of Mr. J. 

 Neilson, of Glasgow, shows some considerable variation in the shape of the posterior 

 end, but as much of this may be due to crushing, and may be apparent rather than 

 real, I will do no more than note the fact. 



With regard to the environment of E. punctatella, Prof. Rupert Jones quotes 

 from a letter of the late Dr. John Young (pp. supra cit., p. 84) : They "exist in 

 thousands both at Darnley (Thornliebauk) and the Linn Spout (Dairy) in a thin 

 shale, in which they seem to be the only organisms present, but in the other shales, 

 lying close to it above and below, we find examples of Pteronites (Actinopteria) 

 and Gypricardia-like shells, having an estuarine or brackish character about them, 

 and immediately below the lower shell is a seam of coal." The idea that the band 



