NO. 9 NEW CAMBRIAN TRILOBITE FAUNULE — RASETTI 7 



SYSTEMATIC DESCRIPTIONS 

 GENERAL STATEMENT 



All the fossils described herein were deposited in the U.S. National 

 Museum collections, excepting one specimen from Quebec which is 

 in the collections of the Geological Survey of Canada. 



All the illustrations show the outer surface of the test. 



The abbreviations (tr.) for transverse, (sag.) for sagittal, and 

 (exsag.) for exsagittal qualify such terms as "wide," "short," etc., 

 whenever the direction of the measurement might otherwise be mis- 

 interpreted. 



Family EODISCIDAE Raymond, 1913 



The Eodiscidae are the most numerous and interesting of the tri- 

 lobites described herein, as they show a greater variety of forms 

 than the genera previously known. Hence the characters observed 

 in the different parts of the exoskeleton are briefly discussed. 



All the species described herein are represented by separate cephala 

 and pygidia; hence the number of thoracic segments is unknown. 



Owing to the large number of species occurring in the same bed, 

 matching of cephala and pygidia presented a problem. In several 

 cases assignments with varying degree of probability could be made. 

 Criteria for associating cephala and pygidia that were used are 

 general shape, size, surface ornamentation, and frequency of occur- 

 rence. It is unlikely that the collection contains a pygidium for every 

 cephalon or vice versa. To avoid the risk of nomenclatural confusion, 

 no taxa were based on pygidia ; those pygidia that could not be 

 assigned to cephala are described and illustrated but not named. 



Features of the various parts of the exoskeleton are discussed 

 below. 



Glabella. — ^The glabella shows considerable variety of form. It may 

 be tapered as in Bolboparia, where it is short in proportion to the 

 length of the cephalon ; more or less parallel-sided as in most of the 

 species ; or even somewhat expanded toward the front as in Serrodis- 

 cus subclavatus and Bathydiscus dolichometopus. Lateral furrows 

 are usually short and shallow or absent. However, in Acimetopus, 

 Analox, and some species of Calodiscus, the glabella is divided by 

 a deep transverse furrow into an anterior and a posterior lobe. A 

 tendency in this direction had been observed, for example, in Calo- 

 discus lohatus and its form agnostoides (Lochman, 1956), in C. 

 helena, C. meeki, and even in Serrodiscus speciosus (Rasetti, 1952). 



