512 



HISTORICAL GEOLOGY. 



The Trilobites include Asaplms platycephalus, Fig. 689; a still larger 

 species, A. megistos Locke, over a foot long, the Ccdymene of Fig. 690, Lichas 

 of Fig. 691, and Trimicleus of Fig. 692. 



The most common species is the Triarthrus Beckii, and the remains 

 usually found are simply the head-shield, represented in Fig. 713. The 



714. 



715. 



716. 



Trilobitkb. — Fig. T14, Triarthrus Beckii, nat. size; 715, a to f (x 8), young' of same, at different stages of 

 growth ; a , the youngest stage (x 15). Fig. 714, Beecher ; 715, a to i, Walcott. 



nearly entire Trilobite, having its tentacles and many of its legs protruded, 

 found as yet at but one locality on the continent, — near Rome, N.Y., — 

 is shown in Fig. 714, from a sketch by C. E. Beecher. Less perfect speci- 

 mens, from the same place, as figured by 

 Matthew, are represented on page 422. 

 The legs of the left and right sides of 

 Fig. 714 are from two dilferent specimens, 

 but are not in any respect "restored." 

 Each has, as made known by Beecher, two 

 branches, and one of them is 

 fringed, and thereby natatory 

 in function. The natatory 

 branch is strictly an append- 

 age to the basal joint of 

 the other branch, which is the 

 true leg. In Fig. 716 A the 

 fringe is removed to show 

 the articulations ; in 716 B 

 the limb is in its entire state. 

 Beecher's observations make 

 certain the close relations of Trilobites to Isopod Crustaceans, as stated on 

 pages 421, 422. 



717. 



Fig:. T16. A, B, leg of Triarthrus Beckii (x 12) ; 

 A, leg with the setae removed to show the 

 articulations , en, the main stem of the 

 leg (endopodite); ex, the natatory branch 

 (exopodite). Beecher. 



Embryonic form 

 of Triarthrus 

 Beckii (x 30). 

 Beecher. 



