PALEOZOIC TIME — LOWER SILURIAN. 



509 



eight inches long ; Calymene (Figs. 690, a) is still more common, — 690 a 

 showing it rolled up, as is often the case (like a modern Oniscus among 

 Crustaceans); 691, aLichas; 692, Trinudeus concentricus (the name referring 

 to the three prominences on the 

 head, and its fillet-like border) ; 

 all are found at Trenton Falls. 

 Another common Trenton species 

 is the Ceraurus pleiirexanthemus 

 Green. Fig. 694 represents an 

 under view of the shell — the ex- 

 uvia of the Trilobite. Walcott 

 states that out of 1160 specimens 

 found by him, only 50 lay with 

 the ba,ck upward, — a natural con- 

 sequence of their being mere empty 

 exuviae, as they would be likely to 

 float like a boat, with the concavity 

 upward. 



Crustaceans of the Ostracoid 

 tribe are not rare. A Leper ditia 

 is represented in Fig. 693. 



8. Fishes. — Remains of Fishes, 

 the earliest knoivnVertebrat.es, occnr 

 in rocks of the Trenton period. 

 The discovery was announced by 

 Walcott in 1891. The fossils are 

 abundant in sandstone near Canon City, Col. Most of them are the 

 plates and scales of Ganoids, the largest about half an inch across. Of 



Teilobite. — Fig. 694, Ceraurus pleurexanthemus, under 

 surface, natural size : 2, tlie hj'postome ; 4, 5, occipital 

 depression and cavity ; a, b, c, d, depressions in the 

 shell of the thorax ; e, free pleurae. Walcott, '75. 



695-697. 



695. 



695 «. 



696. 



697 a 







697. 











; _.3.jiF«: 



%.ir---^ ' : 



' ■- - 







. 1 -: 



- ' -'-^ 





Eemains of Fishes. — Fig. 695, Astraspis desiderata, dermal plate; 695 a, id. (x 3) ; 696, Eriptychius Ameri- 

 canus (x 4) ; 69T, 697 a, Dictyorhabdus priscus, supposed notochord. Walcott. 



