156 CARBONIFEROUS LAMELLIBRANCHIATA. 



teeth. The posterior hinge-teeth have not yet been observed, but there is an 

 elongate ridge just below and parallel with the hinge-line just behind. 



Exterior. — The surface is ornamented by transverse concentric sub -imbricating 

 lines crossed by radiating striae, so that the lower edge of each band is marked 

 by a regular series of points. In front of the valves in adults and in the young 

 the radiating striae are only microscopic, but to the naked eye the concentric 

 lines appear to be finely punctate. There is some irregularity in the concentric 

 lines in the neighbourhood of the byssal sinus. 



Shell moderately thick. 



Dimensions. — Fig. 2, Plate XIII, measures — 



Antero-posteriorly . . . .70 mm. 



Dorso-ventrally . . . .36 mm. 



Elevation of valve . . . .15 mm. 



Locality. — England : the upper beds of the Carboniferous Limestone of 

 Thorpe Cloud, Derbyshire. 



Observations, — This species was established by de Koninck, and he states that 

 it attains to the largest size of all species of the genus Parallelodon. 



I have obtained a fine series of examples in all stages of development from the 

 fossiliferous beds of Thorpe Cloud, Derbyshire, and have been able to work out 

 some of the details of the hinge and muscular insertions. PL XIII, fig. 2, which 

 possesses the greater part of its test in a well-preserved condition, has about the 

 dimensions given by de Koninck, 70, 36, 15 mm., as compared to 74, 34, 14 mm. 

 I have not been able to give the exact dimensions of my specimen in the antero- 

 posterior diameter, as it is not perfect behind. Specimens of P. obtusus seem to 

 have attained to quite as large a size. That from the Gilbertson Collection of 

 the Natural History Museum, South Kensington, figured on PI. XIV, fig. 1, 

 measures 77, 38, 29 mm. 



Indeed, these two species are very closely allied, and I have been in some 

 doubt as to the wisdom of separating them. However, the ornamentation of P. 

 omatissimus is very distinct, and in the adult the shells are expanded posteriorly, 

 though in the young the contour of the two species is very similar. Another 

 difference between these species is the presence of a very wide and long liga- 

 mental area in P. omatissimus, a feature not so marked in P. obtusus. 



I have not been able to see the posterior hinge-teeth, although in several 

 specimens I have stripped off the shell along the posterior part of the hinge-line. 

 Most of the full-grown examples are semi-decorticatecl, and have evidently been 

 rolled, and do not show the peculiar and characteristic markings. This ornament 

 is very similar to that which obtains in P. concinnus, a much smaller form, with a 

 lanceolate posterior end, and not having the very acutely produced posterior- 

 inferior angle of the species under discussion. 



