SILURIAN ANIMALS. 



325 



to Agassiz, we know nearly as much of the development of Trilobites 

 as of any living crustacean. 



Affinities of Trilobites. — The affinities of this very distinct order are 

 imperfectly understood. Crustaceans are divided into two sub- classes, a 

 higher, Malacostraca (mollusk-shelled or calcareous-shelled), and a lower, 

 Entomostraca (insect-shelled). Now, Trilobites, though belonging to the 

 lower division, or Entomostraca, occupy a position near the confines of 

 the two divisions. More definitely, they probably stand between the 

 Isopods (tetradecapod Malacostracans), on the one hand, and the Pliyl- 

 lopods and Limuloids (Entomostracans), on the other. In general ap- 

 pearance they certainly approach Limuloids (horseshoe-crabs or king- 

 crabs), and these seem to have replaced them in the process of evolution. 

 They are by no means very low in the 

 scale of crustaceans ; their position be- 

 ing near the middle. The larvas of 

 crustaceans, especially of Limuloids, 

 greatly resemble some forms of Tril- 



Fig. 384. 



Fig. 3S6. 



Figs. 384-386.— Silurian Eurypterids : 384. Pterygotus Anglicus. viewed from the under side, 

 reduced in size, and restored: c c, the feelers (antennae), terminating in nipping-claws; o o, 

 eyes; m m. three pairs of jointed limbs, with pointed extremities; n n, swimming-paddles, the 

 bases of which are spiny and act as jaws— Upper Silurian, Lanarkshire (after Henry Woodward), 

 385. Eurvpterus remipes, greatly reduced. 386. Same restored: a, dorsal view; b, ventral view 

 (after Hall). 



obites, and especially the larvae of Trilobites. From early generalized 

 forms somewhat like those represented by Eigs. 382 and 383 there have 



