196 SILURIAN MOLLUSCA. 



This differs from the Avicula emarciata (Conrad) by the large 

 gaping anterior side with its double sinus ; the same separates it 

 from a small variety of the A. Boydii (Conrad) ; it is also more 

 square and less oblique than either of these species. The large 

 size of the anterior lobe also separates it from the Avicula qua- 

 drula (Conrad). 



From the Aymestry limestone of Mortimer's Cross, Aymestry. 



(Col. University of Cambridge.) 



Pterinea megaloba (M^Coy). 



Sp. Char. Obliquely subtrigonal, diagonally tumid from the beak 

 to the posterior end (the curve amounting nearly to a semi- 

 circle in old specimens) ; hinge-line elevated into a compressed, 

 nearly rectangular, broad wing, the angle rather nearer to the 

 beak than to the posterior ventral end, which is obtusely 

 rounded, and to which the margin is nearly straight ; anterior 

 end short, forming a very large rounded lobe ; a shallow con- 

 cavity which defines it from the body of the shell extends from 

 the beak to a little in front of the middle of the ventral margin, 

 where it forms a small sinus ; beaks narrow, prominent, in- 

 curved ; anterior muscular impression very strong in the casts ; 

 no teeth. Width from beak to opposite ventral margin 7 lines, 

 length from anterior to posterior ends 1 inch 1 line, width of 

 posterior end 11 lines, depth of left valve 5 lines. Surface 

 apparently smooth, or marked with fine concentric strise. 



The great size of the anterior lobe is the most remarkable 

 character, and is produced by the byssiferous sinus extending 

 backwards at an acute angle to the hinge-line (about 75°), unlike 

 any other species I know. If it was not for the left valve being 

 so much more convex than the right, the species might have 

 been placed in Cypricardites ; the general form is exactly that of 

 the Pterinea rectangularis (Sow. sp.) Sil. Syst. t. 3. f. 2, from 

 which it seems to differ (nine specimens examined) in the want 

 of the diverging cardinal teeth. 



Not uncommon in the tilestone of Storm Hill, Llandeilo. 



(Col. University of Cambridge.) 



Pterinea Sowerbii (M'Coy). 



Avicula reticulata (Sow.), S. S. t. 6. f. 3 (not of Hisinger nor Gold- 

 fuss). 



Sp. Char. Obliquely ovate, depressed, slightly convex, greatest 

 length along the posterior slope, which is straight and defined ; 

 posterior wing gently arched, scarcely extending beyond the 

 shell, its posterior edge slightly and uniformly concave ; sur- 

 face radiated by slightly irregular obtuse ridges, about their 



