262 CARBONIFEROUS LAMELLIBRANCHIATA. 



I have also placed with a ? Pachydomus longus, de Koninck, as a synonym of 

 C. oblonga. This species differs from the others referred to the genus in 

 possessing anterior umbones, which are markedly prosogyrous. The figure 

 appears to me to be an adult specimen of the species under discussion, somewhat 

 flattened relatively to its other dimensions. There is a great tendency for 

 the valves of this fossil to slip on each other, and in consequence the extreme 

 edges are often wanting, and this gives an erroneous idea of the real dimensions 

 and shape. 



This species is easily distinguished from G. ventricosa by its regularly quadrate 

 form, and the absence of the broad deep sul cations which pass concentrically across 

 the shell of the latter. It is not so compressed or transversely produced, nor 

 so oblique as its variety, C. communis. 



Caediomoepha communis, de Koninck, 1885. Plate XXII, fig. 2. 



Cardiomorpha communis, de Koninck, 1885. Ann. Mus. Eoy. d'Hist. Nat. 



Belgique, vol. xi, p. 10, pi. ii, figs. 1,2. 



— speciosa, de Koninck, 1885. Ibid., p. 12, pi. ii, figs. 5, G. 



— parallela, de Koninck, 1885. Ibid., pi. ii, figs. 7, 8. 



Specific Characters. — The general characteristics are as in C. oblonga, and the 

 only difference is one of shape. Shell transversely elongated, subovate, obliquely 

 swollen; narrow in front and expanded behind. The anterior end is very narrow 

 and very little produced ; its border bluntly rounded. The inferior border is 

 nearly straight, and is directed downwards and backwards. The posterior border 

 is obliquely truncate, the postero-superior and inferior angles being well rounded. 

 The shell is moderately gibbose, and the gibbosity of small extent and oblique, 

 posterior to which the valve is rapidly compressed. The umbones are not very 

 large ; prosogyrous and almost terminal. The posterior part of the hinge-line is 

 long and almost straight. The greatest thickness is about the centre of the valve 

 from above downwards. 



Dimensions. — Fig. 2, PL XXII, a somewhat imperfect specimen, measures — 



Antero-posteriorly . . . .62 mm. 



Dorso-ventrally . . . .55 mm. 



Laterally . . . . .43 mm. 



Locality. — The Carboniferous Limestone of St. Dooghlas, Dublin. 

 Observations. — Longer, less globose, more oblique than G. oblonga ; its short 

 anterior and long posterior end give a very characteristic shape to this variety. 

 Fraipont in de Koninck's work says it is closely allied to C. speciosa, with which 

 species and also with G. parallela I believe it to be synonymous. I have only 

 come across a single example in Great Britain, and at present cannot speak with 



