•134 HALKYARD, Fossil Foraminifera of the Blue Marl 



.1881. de la Harpe, P.— Description des Nummulites appartenant 

 a la Zone inferieure des Falaises de Biarritz, des en- 

 virons de la Villa Bruce jusqu' a Handia. 

 Ibid. Ann. 6, 1881, pp. 27-40. 

 — —Description des Nummulites des Falaises de Biarritz 

 Additions et Corrections. Ibid. pp. 229-243. 



1903. Douville, H. — Sur le terrain Nummulitique a Biarritz et 

 dans les Alpes. 

 Bull. Soc. Geol. France, Ser. IV, Vol. II, 1903, pp . I4Q _ 

 154- 

 1905. Douville, H. — Le terrain Nummulitique du Bassin de 

 l'Adour. 

 Bull. Soc. Geol. France. Ser. 4, vol. V, 1905, pp. 9-55. 

 Prever. P.L.-Terreni Nummulitici di Gassino e di Biarritz. 

 Att. Ace. R. d. Sci. Torino Ann. 1905-6, vol. XLI, 

 (pp. 17.) 

 1908. Boussac, J. — Note sur la succession des Faunes Num- 

 mulitiques a Biarritz. 

 Bull. Soc. Geol. France, Ser. 4, Vol. VIII, 1908, pp. 

 237-255. H-A. & E.) 



Ltenus Nummulites, Lamarck. 



Before passing on to speak of the particular species of this 

 genus found in the Blue Marl it will not be out of place to say 

 a few words on the present state of our knowledge as regards 

 the dimorphism of the group. It has long been recognised 

 that there occurred in all Nummulitic deposits certain "couples" 

 of Nummulites, for example where N. variolaria, Sowerby, is 

 found it is always associated with N. heberti, d'Archiac, though 

 for reasons stated below the latter form was not always recog- 

 nised. The only external difference between these two forms 

 (it must be borne in mind that these two forms are only taken 

 as examples of many other couples and to make my statements 

 more clear) is that the latter is generally superior in dimensions 

 to the former. This of itself would not attract much remark as 

 of course the size of a given species of Nummulites depends 

 much upon its age, and is consequently very variable. A further 

 examination of the two forms is therefore necessary. On split- 

 ting the shells in the spiral plane it will be found that the central 

 or primordial chamber of N. variolaria is large and spherical 

 whilst that of N. heberti is extremely minute and in fact hardly 

 distinguishable. The two forms are known respectively as the 

 "megalospheric" or "A" form and the "microspheric" or "B" 

 form. A similar dimorphism occurs in other Orders and 

 Genera of the Foraminifera. There is no doubt that the 



