200 CRETACEOUS LAMELLIBRANCHIA. 



slope of the shell, ;md soiiietiuies for a short distance in front of it, are from lU to 1-j 

 (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 ])ostcrior jiai-t with radial I'ibs, which is 

 serrate. 



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 Ijc 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 Cenoraanian figured by d'Orbigny the shell is i"ather 

 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 

 Cenomanian of Soiith Devon. One specimen from the Cenomanian of Sarthc, 

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

 Blackdown examples. 



The Cenomanian form figui'ed 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. 



Protocar(Jia hifrons (Reuss) is more rounded than P. Ilillana, but does not seem 

 to be specifically distinct. 



Gdvdium mnrticense, Matheron, and ('. llequienianum, Matheron, were regarded 

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



Specimens from the Trichinopoli Group of Soutliern India weir identified with 

 ]\ Ilillana by Forbes and by Stoliczka, who stated that they were unable to draw 

 any line of separation between the Indian and Kui-opean examples. Tlu> 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 specinuMis from Blackdown.' 



• Sec Stoliczka's fif,'. I0,r. 



