200 CRETACEOUS LAMBLLIBRANCHIA. 



slope of the shell, and sometimes for a short distance in front of it, are from 10 to 15 

 (usually 12 or 13) strong, more or less angular ribs, separated by broad furrows, 

 both being crossed by well-marked growth-ridges. Sometimes some of these ribs 

 are divided at their summits by a narrow, longitudinal groove. The ribs become 

 rather smaller dorsally and are absent near the postero-dorsal margin. Internal 

 margins of valves smooth, except the posterior part with radial ribs, which is 

 serrate. 



Measurements : 



















(i) 



(2) 



(•'') 



(4) 



(5) (G) (7) (8) 



(9) 



(10) 



(ii) 



(12) 



Length 51 



48 



44 



44 



43 41 38 36 



36 



32 



27 



19 mm 



Height 47 



47 



44 



42 



41 39 36 36 



(1—12) Blackdown. 



34 



29*5 



26 



18 „ 



Affinities. — Although this species has a very wide geographical distribution 

 and a long range in time, yet the principal variations seen in specimens found 

 at different horizons and in different kinds of sediment consist in the number and 

 coarseness of the concentric ribs. The modifications do not appear to be more 

 than varietal, and such as would be found at the present day in examples of a widely 

 distributed species. 



In the example from the Cenornanian figured by d'Orbigny the shell is rather 

 higher and the ornamentation coarser than in specimens from Blackdown, but in 

 the latter respect it agrees with examples found by the late C. J. A. Meyer in the 

 Cenornanian of South Devon. One specimen from the Cenornanian of Sarthe, 

 shown in Gueranger's photographic illustrations, agrees in its ornamentation with 

 Blackdown examples. 



The Cenornanian form figured by Romer (1870), and the examples from 

 higher horizons figured by Geinitz (1843) and by Goldfuss agree closely with Black- 

 down specimens. Coarser ribbing is found in specimens from the Gault of Cosne, 

 showing that that character is not limited to examples from horizons above the 

 Blackdown Greensand. 



Protocardia bifrons (Reuss) is more rounded than P. Hillana, but does not seem 

 to be specifically distinct. 



Cardium marticense, Matheron, and G. Bequienianum, Matheron, were regarded 

 by d'Orbigny and by Zittel as synonyms of P. Hillana. 



Specimens from the Trichinopoli Group of Southern India were identified with 

 P. Hillana by Forbes and by Stoliczka, who stated that they were unable to draw 

 any line of separation between the Indian and European examples. The concen- 

 tric ribbing is coarser in most of the Indian forms, and in some the smooth inner 

 portion of the posterior area is relatively larger than in specimens from Blackdown. 1 



1 See Stoliczka's fig. 10a. 



