Vol. 68.] IXOCEBAMUS IN THE CEETACEOUS PERIOD. 



19 



/. undulato-plkatus var. digitatus Schliit.^ has the posterior ribs 

 more strongly developed than in /. imdulato-jtlicatus. 



Figs. 93. /. tubercidatus Woods, Actinocamax-quadratus Zone ; right valve. 

 94. /. undulato-pUcatus Rom., Upper Chalk ; right valve. Half of 

 the natural size. 



It is interesting to note that radial folds have been developed 

 independently in several groups of Cretaceous Inocerami, namely : 

 (1) /. concentricus var. suhsulcatus, I. sulcatus ; (2) I. cordiformis ; 



(3) /. suhcardissoides, I. digitatus, I. pinniformis, I. corrugatus ; 



(4) /. cardissoides, I. iuherculatus ; and (5) I. undulato-plicatus. 



In conclusion, I wish to express my thanks to Dr. H. P. 

 Blackmore, Mr. R. M. Erydone, Mr. C. P. Chatwin, Mr. G. E. 

 Dibley, Mr. W. HiU, Dr. A. W. Rowe, and Mr. C. D. Sherborn for 

 the loan of numerous specimens, without which this paper could 

 not have been written. I am also indebted to Mr. C. P. Chatwin 

 for placing his extensive knowledge of the zonal distribution of the 

 Chalk Inocerami at my disposal, and for help in other ways. 



The figures illustrating this paper have been drawn by Mr. T. A. 

 Brock. 



Palseontograpbica, vol. xsiv (1877) p. 267 & pi. xxxvi. 



C2 



