320 



PALAEOZOIC SYSTEM OF ROCKS. 



Fig. 370. 



Figs. 36S-370.— Silurian Cephalopods: 368. Trocholites Ammonins (after Hall): a, exterior; 6, 

 cast, showing septa. 369. Lituites Graftonensis (Meek and Worthen). 370. Lituites cornu-anetis. 



Articulates — Worms. — These are fleshy animals without skeletons, 

 and are therefore not preserved. They are known only by their tracks, 

 their borings, their tubes, and, more rarely, their teeth. Nevertheless, 

 some 185 species, according to Barrande, have been described from 

 the Silurian of different countries. Fig. 371 represents worm-tubes, 

 Fig. 372 worm-tracks, and Fig. 373 worm-teeth, from the Silurian. 



Crustacea — TriloMtes. — The principal representatives of the articu- 

 late department in Silurian times were Crustaceans, but mostly of a 

 very characteristic order of that class, now long extinct, viz., TriloMtes. 



General Description. — The carapace or shell of these curious creat- 

 ures was convex and usually smooth above, and flat or concave below, 

 and divided transversely, like most Crustacea, into a number of movable 

 joints. Several of the front joints are alivays consolidated to form a 

 head-shield or Buckler, and sometimes a number of the posterior joints 

 are similarly consolidated to form a tail-shield or Pygidium. The 

 whole shell or carapace is divided longitudinally, more or less distinct- 



