Fig. 1501. 



7.g ARTHROPODA phylum vii 



ohserved except in Neolimulus. Abdomen trüohed, its segments free, the pleura flat 

 and extended, and usualUj terminating in lateral projeäions or spmes. 



With the exception of the Caml.rian 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 farailies, although several such have been proposed by Packard. Zittel united 

 theni, together witli certain genera of Xiphosura, in the family Henuaspidae, which term is 

 retained, but employed in a restricted sense. 



Suborder A. AGLASPINA Walcott. 



Body elongate, transversely trilobed, more or less sharply divided into two regions 

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

 epistoma and five pairs of movahle appendages. Abdominal 

 Segments all free, varying from seven (Aglaspis) to twelve 

 {Emeraldella) in addition to the caudal spine. 



Family 1. Aglaspidae Clarke. 



Cephalothorax moderately large, trilobed ; abdominal Seg- 

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



Aglaspis Hall (Fig. 1501). Cephalothorax relatively 



large, its trilobed central portion short and conate, in front 



Agias^pu eatoni whitf. of which are two approximate Compound eyes ; bounded 



Ä^ns^r^^/r ' ^'"^'' ^" ^^^ ^^^^^ ^y ^ 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, Habelia and Emeraldella. 

 They are remarkable for displaying well-preserved 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 differences. 



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 The original definition of this family is no longer applicable, 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 



