LAW OF SPECIALIZATION OF PHYLETIC BRANCHES 215 



of the whorls reach their maximum of frequency 

 with the Leptocerata, the Hamulince, the Criocerata, 

 the Ancylocerata, the Macroscaphitce, the Heteroce- 

 rata of the Neocomian, and the Scaphitce, Turrilitce, 

 Bostrychocerata and Baculitce of the upper Chalk. 

 A few palaeontologists, Steinmann in particular, 

 have indeed attempted to give a mechanical expla- 

 nation of the uncoiling of the Ammonites by main- 

 taining that the shells that are much coiled occupy 

 the smallest possible space without having their 

 freedom of motion impeded. But this explanation 

 seems very nebulous, and it would be better to 

 consider this uncoiling as a phenomenon of special- 

 ization carried to the extreme or to look upon it, 

 with von Mojsisovics, Pompecky, and Hyatt, as a 

 senile phase in the evolution of the branches. 



The Lamellibranch Molluscs of the family of the 

 Chamacae will give us, in the Secondary epoch, an 

 example of specialization not less remarkable and 

 quite as incapable of a mechanical or physiological 

 explanation. The Chamae are unequal- valved shells, 

 with lamella ted epidermis, with their tops slightly 

 crooked, which have existed without any great 

 changes from the Chalk down to our seas of 

 to-day. But, from the upper Jurassic epoch and 

 without its being possible for a moment to recognize 

 in them any ancestral forms we see appear a 

 branch of these Chamacae, the Dicer atince, composed 

 of shells large in size, thick, with spiral tops 

 like rams' horns, and with a large cardinal tooth in 

 the form of a gutter. These Dicer ata people the 

 coral reefs till the end of the Jurassic times. From 

 the lower Cretacean, the branches of the Chamacae 



