236 



in front : ventral margin also nearly straight behind the middle, but 

 obliquely as well as rather abruptly rounded upwards in front. 

 Umbones gibbous and very prominent, placed a little in advance of 

 the middle : beaks distant, curved strongly inwards and downwards, 

 with a slight inclination towards the anterior end : ligamental area 

 deeply and angularly excavated, narrowly subrhomboidal in outline as 

 viewed from above : posterior area obliquely flattened, margined by an 

 obtuse and not very well denned keel. Surface marked with crowded 

 and rather irregularly disposed concentric striations, which are 

 crossed, except upon the posterior area, with numerous radiating raised 

 lines. These latter are most prominent and distinct on the anterior 

 third of the shell. The ligamental area, also, is closely and concen- 

 trically striated. 



Hinge teeth and muscular impressions unknown. 



Dimensions of the only specimen collected: length, eighty-seven 

 millimetres ; height, fifty-six mm. ; approximate thickness through 

 the closed valves, sixty-four mm. 



East end of Maud Island : a perfect and well preserved right valve. 



The specimen evidently belongs to a new and interesting species of 

 Trigonoarcd, which can be easily distinguished from the type of 

 Conrad's genus of that name, the Area Ligeriensis of d'Orbigny, (*) 

 by its very prominent umbones and distinct and numerous radiating 

 raised lines. 



MYTILUS LANCEOLATUS, J. Sowerby. 

 Plate 31, figs. 7 and la. 



Mytilus lo,nceolatus, J. Sowerby. 1823- Mineral Conchology, Vol. 5, p. 55, pi. 



439, fig. 2. 

 Mytilus edentulus, " 1823. Mineral Conchology, Vol. 5, p. 55, pi. 



439, fig. 1. 

 Mytilus tridens, 1836- In Fitton's paper in the Translations , 



of the Geological Society of London, Vol. 



4, Second Series, p. 342, pi. 17, fig. 14. 



Mytilus prselongus, " 1836. I b., p. 342, pi. 17, fig. 15. 



Mytilus lanceolatus, d'Orbigny. 1844. PaMontologie Francaise, Terrains Cre- 



taces, Vol. 3, p. 270, atlas, pi. 338, fig. 6. 

 Mytilus lanceolatus, Pictet. 1864-67. Paleontologie Suisse, Terrain 06- 



tac6 de Ste. Croix, Vol. 3, p. 485, in which 



a full synonymy of the species may be 



found. 



Shell obliquely compressed above, strongly carinated below the 

 middle, especially in front, and excavated at the base, in such a manner 



* Pal6ontologie Fran 5 aise. Terrains Cr6tac&. Vol. 3, p. 227, pi. 317. 



