liv 



SYNOPSIS OF GENERA. 



Diagnosis. Carapace hi valvular, each valve obliquely 

 sub-ovate in outline: hinge-line straight and shorter 

 than the greatesi length of the shield. Anterior ex- 

 tremities slightly gaping, posterior extremities some- 

 what produced, incurved or truncate. Rostrum absent 

 or not observed. The cephalic and anterior portion of 

 the thoracic area marked by various regularly and sym- 

 metrically arranged nodes, two of which bear the optic 

 spots. Nuchal furrow more or less distinct. The tho- 

 racic portion of each valve bears one or two longitudi- 

 nal, somewhat curving lateral ridges or carina?. Abdo- 

 men naked, composed, so far as known, of six somites, 

 which are furnished with short spinules or prickles on 

 their dorsal surface. Caudal plate short and produced 

 into a stout telson. Lateral spines or cercopods nar- 

 row and longer than the telson. 



There are now known seven species referable to 

 this genus in which the carapace has been observed, and while these are 

 well-defined by differences in outline and the development of the nodes and 

 ridges upon the surface, it appears that the nodes are constant in number and 

 disposition in all the species. Of these nodes the cephalic region bears two, the 

 anterior of which is large, irregularly defined and sometimes strongly tubercled 

 upon its surface (E. punctata, E. socialis), the posterior small and situated close 

 upon the hinge-line; the thoracic region also bears two, one at the hinge-line 

 just behind the nuchal furrow, obliquely and anteriorly produced ; the second 

 originating near the ventral extremity of the first, transversely and posteriorly 

 elongated and sometimes continuous with the lateral carina. (E. subhevis, 

 E. pustulosa.) 



It is important to notice that certain species of the genus Aristozoi-, Barrande, 

 exhibit an arrangement of the cephalothoracic nodes very similar to that in 

 Echinocaris (A. memoranda, A. regina, A. bisulcata), while other species included 

 under that genus are widely dissimilar in this respect. None of the carapaces 



