Aviculopecten media, sp. nov. Of the type of A. pro- 

 fundus, de Kon., 1 and A. squamuliferus, Morris. 2 



Specific Characters :— A medium sized, nearly circular 

 shell, equilateral without the wings, and nearly equally 

 convex, the right valve being slightly the larger, with the 

 umbo elevated above the hinge. The right posterior auricle 

 is large and almost rectangular, slightly rounded on the 

 outer margin, and ornamented by 9 or 10 low rounded radii, 

 crossed by faint wavy growth lines. Hinge line nearly 

 equal to greatest width of shell. Left posterior auricle 

 large and indented with a marked byssal sinus. Anterior 

 auricles not perfect, but ornamented also with radiating 

 striae. Posterior and anterior slopes equal and fairly steep, 

 but not ridged. Auricles distinct. The test is thin, and 

 ornamented with about 25 equal, rounded costae, which 

 are also prominent on the internal cast. Intercostal spaces 

 broad and smooth. 



Dimensions :— The largest specimen found was 1\ inches 

 in depth; and the dimensions of a nearly perfect cast are: 

 length, 1^ inches, height If inches, thickness of united 

 valves f of an inch. 



Relations and Differences.— This species undoubtedly 

 approaches very closely to A. profundus, de Kon., but after 

 careful examination, I have no doubt that it is distinct. 

 The chief points of difference may be summed up as follows: 



(1) The number of costae in A. media is only about 25, 



instead of 40 as in A. profundus. 



(2) In A. profundus the radii are crossed by numerous 



growth lines, producing a cancellated surface. The 

 surfaces of the radii in A. media are quite smooth. 



' Eth. fil. et Dun, Monograph on Carb. and Permo-Carb. Invert., Vol. 

 n. pt. 1 The Palaeopectens, p. 9, pi. 11, f. 4 and 5. 



• Strzelecki's Phys. Descr. N.S. Wales, 1845, p. 278, pi. 14, f. 1. 



