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SYNOPSIS OF GENERA. 
Diagnosis. Carapace bivalvular, each valve obliquely 
sub-ovate in outline; hinge-line straight and. shorter 
than the greatest 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 carinm. 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. sublcevis, 
E. pustulosa.) 
It is important to notice that certain species of the genus Aristozoe, 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 
