PARALLELODON LACORDAIRIANUS. 147 



re-figuring the shell. In this description he says, " posterior diagonal ridge 

 rounded; surface equally cancellated by coarse, radiating, and transverse sulci; " 

 and " the reticulations on the posterior slope are similar to those on the body of 

 the shell." Why should he have altered the " anterior [posterior] extremity 

 sloped and angular" of Martin into the " posterior diagonal ridge rounded " if, as 

 I take it, " angular " referred not to the border, but to the slope ? 



M'Coy described the posterior end as rectangular ; but I am of opinion, from 

 observation of the shell figured, PI. X, figs. 5, 7, and 9, that this is not quite 

 correct, the angles in adult specimens not being right angles. It is to be noted 

 that the smaller of these two shells is more nearly rectangular than the larger one, 

 and that in young shells such was the shape of the posterior extremity. Martin's 

 type is certainly not rectangular. 



Casts have the radiating strige much more strongly marked over the surface of 

 the valve, especially just anterior to the angulated oblique keel, PI. X, figs. 6, 8, 

 and 9 ; and this fact distinguishes casts of this species from all others of the genus. 



Judging from the figure and description, the shell used for the diagnosis of 

 Area Lacordairlaiia, de Koninck, in 1842, belongs to the species under discussion. 

 The acute posterior inferior angle, strong oblique keel, and the comparative 

 measurements, are very characteristic. Parallelodon comoides of the same author 

 is very probably a synonym of the same species. 



Fig. 9, PI. X, is a small specimen preserved in the Museum of the Geological 

 Survey, Jermyn Street, from Park Hill, which shows the posterior teeth of the 

 hinge, an enlarged view of which is given in fig. 9 a. These teeth are also to be 

 seen in fig. 6, PI. X. 



Pakallelodon Lacordaieianus, de Koninck, 1885. Plate X, fig. 10. 



Non ? Aeca Lacobdaieiana, de Koninck, 1842. Anim. Eoss. Terr. Carb. Belgique, 



p. 119, pi. ii, tigs. 14 a — c. 

 Paeallelodon Lacoedaieeanus, de Koninck, 1885. Ann. Musee Roy. Hist. ]Sat. 



Beige, vol. xi, p. 157 ; pi. xxiv, figs. 48, 49 ; 

 pi. xxv, figs. 19 — 21. 



Specific Characters. — Shell transversely rhomboidal, evenly swollen, of medium 

 size. The anterior end is very short and evenly rounded in contour. Its border, 

 almost straight above, becomes broadly rounded, and passes into the inferior 

 border, which is regularly curved with the convexity downwards, but interrupted 

 a little anterior to its centre by a byssal notch. The posterior end is straight and 

 obliquely truncate from above downwards, joining the lower border by a rounded 

 obtuse angle, and making an obtuse angle also with the hinge-line above. The 



