776 AKTHROPODA phylum vii 



observed except in NeoUmidm. Abdomen trUobed, its segments free, the pleura jlat 

 and extended, and usually terminating in lateral projections or spines. 



With tlie exception of the Cambrian Aglaspis (Fig. 1501), all the genera belonging to this 

 order are of Silurian age, and are too imperfectly known as yet to permit a satisfactory 

 grouping into families, although several such have been ]iroposed by Packard. Zittel united 

 them, together -with certain genera of Xiphosura, in the family Hemiaspidae, which term is 

 retained, but employed in a restricted sense. 



Suborder A. AGLASPINA Walcott. 



Body elongate, transversely trilohed, more or less sharply divided into tioo regions 

 only. Cephalothorax with or without sessile eyes; on the ventral side it has an 

 epistoma and five pairs of movable appendages. Abdominal 

 segments all free, varying from seven (Aglaspis) to tiuelve 

 {Emeraldella) in addition to the caudal spine. 



Family 1. Aglaspidae Clarke. 



Cephalothorax moderately large, trilohed ; abdominal seg- 

 ments with distinct axis and pleurae ; telson long and spiniform. 



Aglaspis Hall (Fig. 1501). Cephalothorax relatively 



Fig. 1501. large, its trilobed central portion short and conate, in front 



Aglaspis eatoni whitf. of which are two approximate compound eyes ; bounded 



w?s^cOT^in"'%'r ' ^°'^'' o^^ ^^^ sides by a distinct border. Abdominal segments 



described as seven in number, flat and blade -like, not 



grooved on the pleura ; telson a long and somewhat obtuse spine. Cambrian; 



Wisconsin. 



Under this family also are included three genera from the Burgess shale 

 member of the Stephen formation (Middle Cambrian) of British Columbia, 

 described by Walcott under the names of Molaria, Ilahelia and Emeraldella. 

 They are remarkable for displaying well-jDreserved abdominal appendages. 



Suborder B, BUNODOMORPHA, nomen novum. 



This group contains only the family Hemiaspidae, as at present constituted. 

 It is a somewhat heterogeneous assemblage, but recognised as separated from 

 the Aglaspidae by more than family dift'erences. 



Family 1. Hemiaspidae Zittel. 



This family, in its restricted sense, may be provisionally maintained 

 pending further investigation of the rare and in some respects obscure forms 

 embraced by it. The original definition of this family is no longer applicable, 

 its limits having become narrowed by the removal of various genera to 

 other groups. 



Neolimulus Woodw. Cephalothorax short and broad, crescentic, elevated 

 mesially, and bearing one or two pairs of ocelli. Compound eyes lateral and 

 connected with the genal angles by a suture. Abdomen very broad anteriorly, 

 not distinctly divided into two regions, all of its segments free, trilobed and 



