LAW OF SIZE IN THE PHYLETIC BRANCHES 195 



any fossil specimen discovered has attained the 

 maximum growth of the species it represents, 

 or if it was still capable of growth in the course 

 of its individual evolution. This is very fre- 

 quently the case with the shells of the Nautilids 

 or of the Ammonites, which construct successive 

 dwelling-places in the course of their growth, and 

 whose characteristics of senility, obliteration of 

 ornamentation, and irregular coils are not always 

 easy to recognize. 



Notwithstanding these difficulties, the law of 

 progression in size is yet verified in a certain number 

 of phyla among the Invertebrates. In the order 

 of Foraminifera, we may cite the phyletic branch 

 of the Orbitolinse, which develops itself from the 

 Barremian to the Cenomanian. In the Barremian 

 and the lower Aptian are found small Orbitolinae 

 a few millimetres in diameter, to which has been 

 given the name of Orbitolina conoidea. At the 

 terminal extremity of the branch in the Cenomanian, 

 the Orbitolinae are represented by the Orbitolina 

 concava, having a very flattened form, and capable 

 of attaining nearly three centimetres in diameter 

 a gigantic dimension for a Foraminifer. Between 

 these extreme forms there are observed in the upper 

 Aptian and in the Gault, particularly in the moun- 

 tains of La Clape and the Corbieres, a whole series 

 of intermediate mutations of size, increasing as they 

 get higher up, which render quite illusive and arti- 

 ficial any specific separation between the Orbitolina 

 discoidea and the concava. The Foraminifera of the 

 Orbitoi'dae family, and, in particular, the Lepido- 

 cyclinse of the Oligocene, offer us a similar progres- 



