PALEOZOIC TIME — LOWER SILURIAN. 



513 



718. 



■CrBEiPEDS. — Fig. 718, 

 Turrilepas Cana- 

 densis, a single 

 plate (X 5). 



Under Fig. 715 are figures of the young Trilobite at different stages of 

 growth, as made out by Walcott — all magnified three times excepting a', 

 which is the stage a magnified 15 times. In this young stage the thorax 

 has but one thoracic segment, and this has a short spine on the back ; the 

 following five segments are abdominal. The other figures {b to i) have an 

 increasing number of thoracic segments. Walcott figures 12 of these stages 

 of growth below the adult, and nine are here reproduced. Beecher has 

 observed a still younger stage having no thoracic segment, represented, mag- 

 nified 30 times, in Fig. 717. 



Other genera of Trilobites of this epoch are Ceraurus, Acidaspis, Proetus, 

 Dalmanites, and Cyphaspis. 



Besides Ostracoids of several genera, there were also 

 the first known species of the Barnacle or Cirriped tribe 

 — the Turrilepas Canadensis Woodward. The specimen 

 figured (Fig. 718), rep- 

 resenting one of the 

 pieces of the shell, was 

 from near Ottawa, Can- 

 ada. 



The Utica slate has 

 afforded the first speci- 

 mens of the Eurypte- 

 rids — species remotely 

 related to Crustaceans, and peculiar in 

 having five pairs of large legs projecting 

 either side of the head whose basal joints 

 serve as jaws (page 556). Fig. 719 rep- 

 resents a leg of one of the pairs ; and as 



it is half the natural size, the whole animal was probably more than a foot 

 long. Its fringe of spines aided it in swimming, and perhaps also in securing 

 its food. Entire specimens of other species of the tribe are shown on pages 

 556, 564. 



Characteristic Species. 



1. Trenton Epoch. 



1. Spongiozoans. — Beceptaculites Oweni H., characteristic of the Galena limestone, 

 with B. globularis H., B. lowensis Owen. Astylospongia parvula Bill., near Ottawa City, 

 Canada ; JBrachiospongia dirjitata (Fig. 642) is from a paper by C. E. Beecher, which is 

 illustrated by 6 plates, published by the Peabody Museum of Yale College. The species 

 was first described and figured by Troost in 1839 ; named Scyphia digitata by D. D. Owen 

 in 1858, and Brachiospongia Boemerana by Marsh in 1867. Beecher also describes in 

 the same paper two other species of Sponge under the generic name Strohilospongia ; 

 they occur with the preceding. The most recent observations of Rauff make the supposed 

 relations of the Beceptaculites to the Sponges very doubtful. 



2. Actinozoans. — Pig. 644, Streptelasma corniculum H., S. profundum Con., Trenton 

 limestone; S. aptertura B., Black River limestone. Fig. 645, Columnaria alveolata Goldf., 

 Black River limestone, and Trenton ; C. Halli Nicholson, Kentucky ; C. calicina Nicholson, 



DANA'S MANUAL — 33 



Fig. 



719, Leg of Echinognathus Cleveland!. 

 Walcott. 



