138 CARBONIFEROUS LAMELLIBRANCHIATA. 



folds, the lower edges of which are raised into rib-like processes, more pronounced 

 on the right valve than on the left, and becoming wider apart towards the lower 

 border of the shell. There is no trace of radiating lines, as a rule, or of zigzag 

 markings. 



Dimensions. — PI. IX, figs. 1 a, b, measures — 



Antero-posteriorly . . . .19 mm. 



Dorso-ventrally . . . .11 mm. 



Laterally . . . . .8 mm. 



Localities. — England : the Carboniferous Limestone of Trey Cliff, Castleton, 

 and Thorpe Cloud, Derbyshire. Ireland : Lower Limestone ; Howth. 



Observations. — I am permitted by the kindness of Dr. Scharff to re-figure the 

 original specimen of M'Coy's Venerupis cingulatus, now in the Griffith Collection 

 of the Museum of Science and Art, Dublin. M'Coy's description is " transversely 

 elongate, oval, cylindrical, very convex ; beaks close to the anterior side, which 

 is very small, rounded ; posterior side elongated, obliquely truncated, rounded ; 

 surface with about ten or eleven large, angular, concentric ribs, which are some- 

 times crossed by fine longitudinal striae." He remarks, "In this species the 

 concentric ribs are very regular ; they are large, angular, and sharp above." 



The type specimen (PI. IX, fig. 4) is not well preserved ; but, although 

 fragmentary, shows very markedly the obliquity of the shell. I can see no 

 traces of radiating striae on the wax-cast that has been sent me for study, and 

 cannot bear out M'Coy's observation that the ribs are " sometimes crossed by 

 fine radiating striae," though such a character might be almost expected in the 

 genus. Mr. Etheridge seems to have been of the opinion that this species was a 

 synonym of Pullastra bistriata (Portlock) ; but the two species are very distinct, 

 and I have indicated the chief points in the different published diagnoses in my 

 " Observations " on that species, p. 135. 



P. cingulatus is the only species of the genus which has unequal valves, and 

 this character is very marked indeed (see figs. 1, 1 a, 1 b, 2, 2a, PI. IX). It will 

 be interesting to note if this character is accompanied by any concomitant change 

 in the details of the hinge. I hazard the suggestion that this character may 

 afford an indication of the descent of the genus Parallelodon from some unequal- 

 valved ancestor, a subject which I have referred to previously, p. 129. 



Parallelodon squamosus, de Koninck, sp., 1842. Plate IX, figs. 18 — 22, 22 a. 



Aeca squamosa, de Koninck, 1842. Anim. foss. terr. Carb. Belgique, p. 121, 



pi. ii, figs. 13 a, b. 

 — fimbeiata, de Koninck, 1843. Ibid, (parmi les corrections), p. 634. 



