62 CRETACEOUS LAMELLIBRANCHIA. 



show a more or less regular alternation in size, but sometimes two or three of the 

 larger ribs occur in proximity without the intervention of smaller ribs. In some 

 cases between the large and small ribs a rib of still smaller size is found. Some of 

 the large ribs start from near the umbo ; others start at some little distance from 

 it but soon reach the same size as the primary ribs ; still other ribs are intercalated 

 at a greater distance from the umbo and do not attain the same size as the 

 earlier ribs. The anterior and posterior ribs have a slight curvature ; the others 

 are more nearly straight. Occasionally the anterior and posterior ribs have a 

 faintly marked nodose appearance. The interspaces are smooth, or have a very 

 faintly marked radial ribbing. The anterior ear is ornamented with ribs similar to 

 those on the remainder of the valve, but they are of uniform or nearly uniform 

 size. The posterior ear is marked with growth-lines parallel to its posterior con- 

 cave border ; radial ribs also occur, and are rather larger and more widely separated 

 on the dorsal portion than on the part near the junction with the rest of the valve. 

 Right valve much smaller than the left ; flattened, but convex in the median 

 dorsal part. Anterior ear with a deep sinus. Posterior ear much larger, but not 

 marked off from the rest of the valve. Surface smooth, or with very faint concen- 

 tric lines. 



Measurements of left valves : 















(1) (2) 



(3) 



(4) 



(5) 



(6) 



(7) 



(8) 



Length . 27 26 



26 



20-5 



19 



19 



1:3-5 



12 mm. 



Height (oblique) 25-5 23 



22 



19 



18 



17 



14-75 



11-25,, 



(1, 8) A. quadratic zone, West Harnham. 

 (2, 5) „ ,, Coddenham. 



(4) Upper Chalk, Ruston Parva. 

 (3, 6, 7) „ Wells. 



Affinities. — Pterin danica (Ravn) x is similar in outline and the general 

 character of its ornamentation to Pterin tenuicostata, but appears to be dis- 

 tinguished by the possession of fewer ribs and by their absence in the neighbour- 

 hood of the umbo. 



In the character of its ornamentation P. tenuicostata closely resembles P. 

 pectinata (see p. 59), but is distinguished by its larger size, relatively greater 

 length, and smaller convexity; also the ribs are more numerous, the ears are 

 relatively larger, and the anterior left ear is less distinctly limited. 



The specimen from Simbirsk figured by d'Orbigny (1845) as Avicula tenui- 

 costata differs from that species in the possession of fewer and stronger ribs. It 

 was subsequently regarded by d'Orbigny 2 as an example of Avicula laripes, Morton. 3 



1 'Mollusk. i. Danmarks Kridtaflej,' i. (1902), p. 79, pi. i, figs. 1, 2. 



2 ' Prodr. de Pal.,' vol. ii (1850), p. 249. 



3 ' Synopsis Org. Remains Cret., U.S.' (1834), p. 63, pi. xvii, fig. 5. 



