NO. 3 CAMBRIAN AND OZARKIAN TRILOBITES 65 



Agnostidae, from which it is hoped much data will be obtained for 

 more exact correlation of the older beds. Several of the studies listed 

 above will be published in parts, which together with descriptions to 

 be published by me from time to time, will in the near future equip 

 the worker in Cambrian stratigraphy so that more detailed and more 

 correct stratigraphic conclusions can be reached. 



The genera described and illustrated in this paper were recognized 

 and prepared for study at various times during the past lo vears. 

 Diagrammatic outline sketches of most of them were published in 

 1924,* and the names given many have been used in connection with 

 description of formations and geological sections.^ In the preparation 

 of descriptions of genera, free use was made of Dr. Ulrich's notes 

 on some ; also the observations of Dr. Resser. 



The dorsal tests of the trilobites are usually dismembered and often 

 only a single head or tail of a species is found ; then again the rock 

 may be almost made up of various parts of many species crowded 

 together in great disorder. The most skillful manipulation is required 

 to work out identifiable specimens and then the interpretation of the 

 probable relations of the various parts requires patient study and a 

 wide acquaintance with the fauna. 



Mr. J. A. Mirguet did much of the preliminary working out of 

 the fossils from their matrix. Miss Sara Evans has assisted Dr. 

 Resser, and Miss Frances Wieser and Miss Doris Cochran have made 

 outline sketches and retouched the photographs where necessary. 



DESCRIPTION OF GENERA AND SPECIES 

 Genus AMECEPHALUS Walcott 



Amecephalus Walcott, 1924, Smithsonian Misc. Coll., Vol. 75, No. 2, 

 pp. 53, 54- 



Description. — The genus Amecephalus was established to include 

 the forms with a wide frontal border that were formerly placed in 

 Ptychoparia piochensis (Walcott) (s. 1.). 



The broad, flat border is characteristic of the genus. Cranidium 

 wide with the glabella occupying only about one-half the length of the 

 head and well defined by the dorsal furrow. There are three sets of 

 glabellar furrows that are usually moderately impressed and which 

 turn sharply backward as they approach the middle of the head 

 where -most of the specimens of the type species have a more or less 



^ Smithsonian Misc. Coll., Vol. 75, Nos. i and 2, 1924. 



^ Smithsonian Misc. Coll., Vol. 67, No 8, 1923 ; Vol. 75, No. i, 1924. 



