186 



PALAEOZOIC TIME LOWER SILURIAN. 



The genera Lingula and Discina are the only two in the animal 

 kingdom, as far as now known, that began with the primal life of 

 the globe, in the Potsdam epoch, and continued on, in a succession 

 of species, to the present time, — the species changing, but not the 

 genera. The analyses of the ancient and modern shells by T. S. 

 Hunt confirm the fact of the family identity between the ancient 

 and modern species (p. 68). Unlike nearly all other shells, they 

 have the constitution of bones, or are mainly phosphate of lime. 

 Another genus, Pleuroiomaria, and also Nautilus, if the specimens 

 are rightly referred, commenced in the Calciferous epoch, and still 

 have living representatives, nearly equalling the Lingula in the 

 length of their history. 



Characteristic /Species. — 1. Potsdam, Epoch. 



Protozoans. — Sponges. — Fig. 236 A, Archeocyathus Atlanticus B., is 

 either a coral or a sponge (Billings) : a re- 

 presents the external form, diminished 

 one-half in size ; b, a polished transverse 

 section (natural size), showing an irre- 

 gularity of structure more like that of a 

 sponge than a coral. It comes from the 

 north shore of the Straits of Belle Isle. 

 A. Minganensis B. is another species from 

 the same locality. 



Radiates. — a. Polyps. — No corals 

 have heen found in this formation, unless 

 the Archeocyathus be of this nature. 



b. Acalephs. — Fig. 244, Graptolithus Hallianus Prout, from St. Croix, Minne- 



Figs. 237-244. 



Fig. 236 A. 



Archeocyathus Atlanticus. 



Figs. 237, 238, Lingula prima; 239, L. antiqua; 240, Scolithus linearis: 241 a, b, Cono- 

 cephalus minutus, head and tail shields (X 4) ; 242, Dicellocephalus Minnesotensis 

 (X %); 243, D. Iowensis; 244, a, Graptolithus Hallianus. 



sota. For an explanation of the nature of Graptolites, see beyond, on page 190. 

 Fig. 244 a is an enlarged view of a branch. 



