164 CARBONIFEROUS LAMELLIBRANCHIATA. 



and backwards, but posteriorly is curved upwards to meet the posterior border, 

 which is very much prolonged above and regularly curved. The hinge-line is very 

 short, straight, and elevated posteriorly ; its junction with the posterior border is 

 almost imperceptible. 



The umbones are anterior, but almost at the centre of the hinge-line, very 

 small, inconspicuous, hardly raised above the hinge-plate, and almost contiguous. 

 There is no area to be seen. 



The valves are regularly and gradually but slightly curved from above down- 

 wards, and from before backwards, the posterior-superior angle being slightly 

 compressed. There is no ridge nor fold, and the most gibbose part of the valve is 

 near the umbo. 



Interior. — The anterior adductor scar is small and round, submarginal, and 

 situated not quite halfway down the anterior border. The posterior adductor 

 scar is small, and situated just below the hinge-plate, but at some distance from 

 the posterior border. 



The hinge-plate is expanded in front and behind, and has elongate subparallel 

 teeth posteriorly, not reaching to the end of the hinge-plate, and small oblique 

 teeth in front. Pallial sinus entire. 



Exterior. — The surface is ornamented with a number of well-marked, sub- 

 imbricating, irregularly placed lines of growth, parallel to the borders ; the spaces 

 between the lines are covered with numerous very fine, regular, microscopic lines. 



Dimensions. — Fig. 30, PI. XI, measures — 



Antero-posteriorly . , . .30 mm. 



Dorso-ventrally . . . .19 mm. 



Laterally (one valve) . . . 3*5 mm. 



Localities. — The Carboniferous Limestone of Castleton, Derbyshire, and 

 Narrowdale, North Staffordshire. 



Observations. — Very unlike a member of the Area family, and, as de Koninck 

 observes (op. supra cit.), " la forme de cette Area, a une si grande ressem- 

 blance avec celle des Garclinia qu'il eut ete difficile de la ranger parmi ses con- 

 generes, si je n'etais pas parvenu a degager une partie de la charniere." I figure 

 an interior, PI. XI, fig. 28, and if I am correct it would appear that in adults the 

 valves became relatively flatter as they expanded in other directions. In the fine 

 specimen from Castleton, PI. XI, fig. 30, I have been able to ascertain that there 

 was an expanded hinge-plate posteriorly ; the details are shown in PI. XI, 

 figs. 28 and 28 a. 



It is curious that in deKoninck's great work of 1885 {op. supra cit.) the species 

 Parallelodon Haimeanus is not described nor figured, nor is this term stated to be 

 a synonym of any other form. A solitary mention, however, is made of that 

 species in the remarks on P.fallax, p. 153, which is stated to resemble that shell 



