Paleontology 685 



Cambrian rocks of Ireland, but differs in some particulars, so 

 that the author considers it a distinct species, and proposes 

 for it the name Oldhamia {Murchiso7iites) occidens. 



Another important work issued during this period is "The 

 Genesis of the Arietidte," contained in the twenty-sixth 

 " Smithsonian Contributions to Knowledge," by Alpheus 

 Hyatt. This memoir forms a quarto volume of over two 

 hundred pages, including introduction, index, and explanation 

 of plates. The general plan of tlie treatise is a discussion of 

 the genealogy, genesis of characteristics, geologic and faunal 

 relations, and, finally, descriptions of genera and species of 

 Arietid Ammonitoidea. 



The work sets forth the successional development of the 

 different types of the ammonites of this division through the 

 successive geologic ages, and is an important contribution to 

 the general doctrine of evolution, since these forms illustrate 

 the nature of direct evolution in definite directions, or ortho- 

 genesis, as it is called. 



The illustrations consist of thirty-five figures in the text, 

 six folding charts or tables, and fourteen plates, of which ten 

 are heliotypes. 



Simpson's papers on " Fossil Unionidct " complete the list 

 of invertebrate publications. These appeared in 1893 and in 

 1896, respectively. The first contains descriptions of Unios 

 and other fresh-water shells from the Drift at Toronto, Can- 

 ada, and a review of the distribution of the Unionidae of 

 northeastern North America. The second paper comprises 

 diagnoses of some new Triassic Unios from the Staked Plains 

 of Texas. The material on which this paper is based was 

 obtained from the Dockum beds, an extensive Triassic fresh- 

 water formation deposited in shallow water, underlying the 

 Staked Plains of Texas. Four species are recognized. Taken 

 as a whole, these Unios closely resemble in form, and are 

 44* 



