298 



GEOLOGY. 



fossil all through the Cambrian was the Hyolithes, usually referred to the ptero- 

 pods. H. americanus (Fig. 120, a) was one of many Lower Cambrian species. 



There were forms usually referred to sponges, but identified by Hinde as 

 corals, such as Archceocyathus renssel&ricus (Fig. 124, a and b) and hydrozoa, 

 such as Phyllograptus cambrensis (Fig. 124, e). Fig. 124, c and d, Brooksella 

 alternata, represent a series of singular forms regarded by Nathorst and others 

 as the casts of the gastric cavity of a medusa. 1 There were worm-borings, tracks, 

 and unidentifiable forms, making up, altogether, a rather varied fauna. 



The Middle Cambrian or Paradoxides fauna. — The general aspect of the fauna 

 of the Middle Cambrian was similar to that of the lower division. There was but 



Fig. 127. — Middle Cambrian Fossils. Trilobites: a, Paradoxides bohemicus Boeck; 

 b, Olenoides curticei Walcott; c, Ptychoparia kingi Meek; d, Agnostus interstrictus 

 White; Brachiopods: e, Protorthis billingsi (Hartt); g, Linnarsonia transversa 

 (Hartt); Gastropods: f, Harttia matihewi Walcott, interior of the shell. 



slight evidence of advance in organization, and as knowledge now stands, no 

 increase in numbers, but apparently a decline, which is probably merely a defect 

 of the record. The Olenellus had disappeared and the Paradoxides (Fig. 127, a) 

 reigned in its stead. The trilobites appear to have reached their climax in size 

 in certain species of the Paradoxides, some individuals attaining a length of a 

 foot and a half, but there was further advance in structure and numbers in the 



1 For a special discussion and numerous illustrations of these forms, see Fossil 

 Medusa?, Mono. XXX, IL S. Geol. Surv., 1898, by Walcott. 



