Geology and Mineralogy. 513 



brian time some had become vegetable feeders and that in the late 

 Cambrian most of them appear to have become omnivorous. A 

 few may have been mud eaters. 



Part III discusses the relations of trilobites to the other groups 

 of the Arthropoda, and the conclusion is reached that the trilo- 

 bites are the most primitive members of the phylum and either 

 directly or indirectly ancestral to all the other orders. The ances- 

 tor of the trilobites is thought to have been a ' ' soft bodied, free 

 swimming, flat, blind or nearly blind animal of few segments," 

 and Naraoia compact a Walcott is suggested as the most primi- 

 tive of all known trilobites and hence closest to the ancestral 

 stock. A diagram showing the interrelationships, time of origin, 

 and duration of the Arthropoda is given in figure 41 on page 

 150. Among the striking phylogenetic conclusions are: (1) that 

 the insects probably had their origin in the trilobites, though not 

 directly, but through some tracheate stock which was directly 

 ancestral to the diplopods and chilopods as well; (2) that the 

 diplopods, chilopods, and the higher Crustacea had their origin in 

 the trilobites back of the Cambrian; (3) that the arachnids, 

 eurypterids, and horseshoe crabs arose in the xenopods (a new 

 subclass of Crustacea), and that this stock also developed out of 

 the trilobites in pre-Cambrian times; (4) that the copepods had 

 their origin in the most primitive of trilobites (Hypoparia), 

 independently from the rest of the higher Crustacea; (5) that 

 the Arthropoda ''constitute a natural monophyietic group," of 

 which the Trilobita are the ancestral, oldest known stock; and 

 (6) that the appendages of all other arthropod lines ''could have 

 been derived from those of trilobites. ' ' 



Part IV gives descriptions of the appendages of individual 

 specimens of Triarthrus hecki Green and CryptoUthus tesselatus 

 Green. 



An excellent bibliography and an historical review of the 

 investigations relating to trilobite appendages are other parts of 

 the work. The forty-six text figures serve their purpose well, 

 and there are in addition eleven plates, of which ten show photo- 

 graphs or drawings made by Professor Beecher or under his 

 direction. On Plate 11 is given an excellent restoration of 

 Ceraiirus pleurexanthemus, drawn by Doctor Elvira Wood. 



W. H. TWENHOFEL. 



2. The Geology of Hardin County, and the Adjoining Fart of 

 Pope County; by Stuart Weller, with the collaboration of 

 Charles Butts, L. W. Currier, and R. D. Salisbury. Illinois 

 Geol. Survey, Bull. 41, pp. 416, 11 pis., 30 text figs., 1920.— In 

 this very detailed county report are described the Devonian, Mis- 

 sissippian, and Pennsylvanian formations, which together have 

 a thickness of over 3,500 feet. The book is particularly valuable 

 because of the detailed description of the Mississippian sequence. 

 The region has been bowed up into an immense dome, then broken 

 into a complex series of blocks and intruded by dikes, sills, and 



