LAW OF SIZE IN THE PHYLETIC BRANCHES 197 



of the Crustacea the interesting little branch of 

 the Limulidse, which takes birth in the Trias with 

 quite small forms, such as the Limulus priscus of 

 the Muschelkalk of Bayreuth, continues in the 

 higher Jurassic with types of average size, like the 

 Limulus Walchi of the lithographic stone of Solen- 

 hofen, then with the Limulus Decheni of the Oli- 

 gocene, and attains the apogee of its dimensions in 

 the L. polyphcemus, still existing in the Gulf of 

 Mexico. 



The lower Vertebrates supply us in their turn 

 with numerous examples of the law of progression 

 in size of each branch. The Sharks of the genus 

 Carcharodon include, in the lower Eocene, species 

 of medium size, like the Carcharodon appendicu- 

 lator of the Tunisian plateau, continue to increase 

 in size in the middle Eocene and the Oligocene, and 

 attain in the Miocene and the Pliocene the astound- 

 ing dimensions of the Carcharodon megalodon, 

 certain triangular teeth of which twelve centi- 

 metres in length indicate a Shark of a total length 

 of twenty metres. There is known in the Primary 

 formations, from the higher Silurian to the Permian, 

 a group of Elasmobranchial Fishes, the Acanthodce, 

 characterized by their firmly shagreened skin and 

 their fins strengthened by a large spiky ray. The 

 representative forms of this group in the red De- 

 vonian sandstone, such as the Acanthodes Mitchelli, 

 are quite small fishes, with a body not exceeding 

 seven centimetres in length. When we reach the 

 Carboniferous, the Acanthodes Wardi grows to 

 twenty-seven centimetres, and certain unequal 

 spines of fishes of this group, known by the name 



