324 



PALAEOZOIC SYSTEM OF ROCKS. 



reached their maximum of development, in size, number, and variety, 

 in the Silurian. Barrande gives the number of species described in the 

 Silurian alone as 1,579. They reached in some cases a size equal to any 

 crustaceans now living. The Asaphus (Isotelus) gigas, from the Lower 

 Silurian (Fig. 380), was sometimes twenty inches in length and thir- 



Fig. 381. — Dalmania limulurus. 

 a 



Fig. 382.— a, Larva of a Trilobite; b, Larva 

 of a King-Crab (af ter Packard). 



Fig. 383.— Limulus before hatching, Trilobite Stage: a, Bide view ; b, dorsal view (after Packard). 



teen wide. Parodoxides (Figs. 289 and 290, p. 301), of the earliest 

 Primordial, attained a length of twenty-two inches. On account of 

 their great abundance and fine preservation, their embryonic develop- 

 ment has been carefully studied by Barrande, who has described and 

 figured twenty steps in the development of some species. According 



