NO 2.] DESCRIPTION OF THE FOSSILS. 61 



fine radial ribs. On some fragments, slight indications of very fine, close, 



concentric grooves are to be seen. 



The right valve is flatter. It shows concentric, 

 leaf-shaped lamellae (fig. 14a), or very faint radial 

 ribs. In pieces that have been weathered out 

 of the rock, the interesting construction of the 



Fig. 9. Pseudomonotis Jacksoni. inside (PI. I, fig. 14b, c, and letter-press fig. 9) 



n. sp. Interior of the specimen 



PI. I, fig. 15, showing the oblique ma y be observed. On the broad area, a shallow, 



ligament groove on the area; triangular ligament-groove extends obliquely from 

 nat. size. 



the umbo towards the back (Letterpress fig. 9). 



Below the area lies a rounded or triangular, deep, cup-shaped groove, 

 growing shallower towards the back. The long incision for the byssus, ex- 

 tending obliquely along the valve, becomes obliquely covered up, to some 

 extent altogether closed by an interior thickening of the valve of the anterior 

 wing. Fig. 14c shows the remarkably thick, swollen margins of the byssus 

 fissure in profile. 



Remarks. Newton compares the form in question with Avicula ince- 

 quivalvis, Sow. from the Callovian, but points out, as a difference, that 

 the ribs are coarser than in Sowerby's species. Apart from the fact that 

 Avicula incequivalvis Sow. of so large a size as the species before us, is 

 unknown, there is also the fact that the left valve of Avicula incequivalvis 

 Sow. is always considerably more oblique, and more elongated. Avicula 

 incequivalvis has, moreover, fewer ribs, and the spaces between the 14 ribs 

 are almost, if not quite smooth. 



The sculpturing of the valves recalls more strongly that of the so-called 

 Avicula Miinsteri Goldf. Apart from the difference in size between the two 

 forms, identification with this species is impossible, on account of the con- 

 struction of the inside of the valve. 



The inside of the right valve, especially in the deep, cupshaped impres- 

 sion (of the anterior muscle), shows some affinity to the subgenus "Melea- 

 grina Lam". A similar deep impression was observed by Fr. Teller, on the 

 Avicula (Meleagrina) Tundrce Tell J , described from the East Siberian Trias. 



1 Fr. Teller, 'Die Pelecypoden-Fauna von Werchojansk in Ostsibirien' (in E. Mojsisovics 

 von Mojsvar : Arktischc Triasfaunen) M6m. de 1'Acad. imper. des sciences de St. Pelers- 

 bourg. Ser. VII. vol. XXXIII, no. 6, 1886, p. 133. pi. XIX, fig. 9. 



