SILURIAN MOLLUSCA. 197 



thickness apart, (five in 2 lines about the middle, at an inch from 

 the beak,) partially interrupted by thin concentric imbrications 

 from 1 to 2 lines wide, having the radiating ridges obsolete 

 or nearly so on their half ; radiating ridges of the wing rather 

 larger, strongly marked only about the middle. Length from 

 beak to respiratory* angle 2 inches 6 lines, length of posterior 

 wing 1 inch 6 lines, width of ditto 1 inch 1 line, wddth from 

 middle of hinge-line to ventral margin 2 inches 5 lines, depth 

 of one valve 3 lines. 



This fine species differs from the Pterinea reticulata of the 

 original continental authors, in its more elongate form, smaller 

 posterior wing, with its gently concave posterior edge, and the 

 comparatively few, broad, thin imbrications interrupting the 

 radiating ridges. 



Aymestry limestone, Leintwardine. 



{CoL University of Cambridge.) 



Pterinea termistriata (M^Coy). 



Sp, Char. Subquadrate, rounded, slightly oblique, evenly gib- 

 bous, left valve most so ; width only slightly exceeding the 

 length ; beaks large, tumid ; anterior wing half the length of 

 posterior one, abruptly compressed, rounded ; ventral margin 

 and posterior end broadly rounded, posterior margin slightly 

 concave towards the cardinal angle of the posterior wing, which 

 is gradually compressed and scarcely extends beyond the shell ; 

 surface with irregular concentric wrinkles of growth crossed 

 by very fine equal or subalternate radiating strise from the 

 beak, strongest in the middle, about six in 1 line, less than 

 their diameter apart ; posterior lateral tooth or hinge-plate as 

 long as the hinge-line, and close beneath it, a thick internal 

 ridge (often leaving a sulcus in casts) curves from behind the 

 beak towards the ventral margin at an angle of about 50° to 

 60° from the hinge-line; hinge- line crenulated, forming Area- 

 like transverse teeth in front of the beak. Width from beak 

 to opposite ventral margin 4 lines, length 4|^ lines. 



This little species is much less transverse, and has a more ob- 

 tuse posterior wing than the Avicula insueta (Conrad), which it 

 otherwise much resembles. The abrupt bending-down of the 

 curved internal ridge, like a lateral tooth, into the body of the 

 shell, is a curious character, in which, as well as general form, it 



* As the term anal angle is commonly used in speaking of the angle 

 between the end of the hinge-line and the posterior margin, I propose 

 using the term respiratory angle in the description for the angle between 

 the posterior and ventral margins — the excretory or anal siphon being next 

 the former, and the inhahng or respu-atory siphon next the latter. 



