84 GUIDE TO THE FOSSIL INVERTEBRATE ANIMALS. 



Gallery lip. Specimens of other species showing these structures 

 'abie'case exhibited. Every body-segment, except that in which 

 25". SiJius opens, bears a pair of appendages, attached to 



transverse thickenings of the ventral membrane. The front 

 pair form whip-like antennae. The remaining pairs are 

 branched, one branch being a crawling leg, the other branch 

 bearing a fringe of bristles or of lamellae. The basal seg- 

 ments of the four pairs on the head served to bite food and 

 to pass it into the mouth. The lamellate branches of the 

 remaining limbs may have served partly for swimming, 

 partly for breathing. 



Trilobites lived only in the sea, some on reefs, some on 



Fig. 39. — Reconstruction of a Trilobite, T7'iarthrus Becki, from the 

 Ordovician, Utica Slate of New York ; natural size. (After Beecher. 

 Table-case 25.) 



muddy or sandy bottoms; some, it is inferred either from 

 the absence or the extraordinary size of the eyes, in deep 

 water. In the growth of an individual trilobite of simple 

 structure, the free cheeks and the eyes borne by them are 

 at first not seen on the upper surface of the head-shield. As 

 the animal grows they appear at the edge, and gradually 

 come to occupy more and more of the upper surface. Some 

 Table-case early trilobites, however, such as Agnostus (Fig. 40 a), 

 25. Harfes, and Trinucleus, never reach this stage, and may be 

 separated as a Grade Hypoparia (under-cheeks) from those 

 in which the free cheeks are visible on the upper surface. 

 In these latter the free cheeks may be confined to the fore- 



