470 



HISTORICAL GEOLOGY. 



1. Lower Cambrian. 



1. Protozoans. — No Rhizoporl remains have been detected, unless small 

 concretion-like nodules, concentric in structure, occurring crowded ly in a 

 Cambrian limestone in Nevada, are of this nature. They may belong to 

 the genus Girvanella (Walcott). See page 501. 



2. Sponges, Corals, Graptolites. — Fig. 506 represents one of the Lower 

 Cambrian sponges, Leptomitus Zittelli of Walcott, from Georgia, Vt. 



Figs. 507, 508 are of corals, though supposed, when described, and until 

 investigated microscopically by Hinde, to be Sponges. Fig. 507 represents the 

 Ao'chceocyathus x>rofundus of Billings, and 508, 508 a, views of Sjnrocyathus 



506-509. 



506. 



507. 



508. 



509. 



508 a. 



Sponge. — Fig. 506, Leptomitus Zittelli. — Corals, 507, Archaeocyathus profundus ; 508, Spirocyathus Atlan- 

 ticua(^); 508 o, transverse section. — Graptolite, 509, Climacograptus (?) Emtnonsi. Figs. 506, 509, 

 Walcott ; 507, 508, Billings. 



Atlanticus Billings. One of the early Grcqotolites (so called from the Greek 

 •ypacfyw, ivrite, because plume-like in form) is represented in Fig. 509, doubt- 

 fully j)laced in the genus Phyllograptus by Walcott, under the name Climaco- 

 graptus ( ?) Emmonsi. 



3. Echinoderms. — Only fragments of Cystoids, related to Middle Cam- 

 brian species, have been observed. 



4. Worms. — Tracks and borings of sea-worms or Annelids are not un- 

 common. Worm-borings, called Scolithus (from the Greek for worm), occur 

 in the Lower Cambrian sandstones and through later periods to the present 

 time : no distinction of species or genera can be made out. 



