798 The American Naturalist. [September, 
ciation with the unicarinate carapaces such as are possessed by 
Rhinocaris columbina, R. scaphoptera, Mesothyra oceani, M. 
(Dithyrocaris?) veneris, Tropidocaris hamiltonim, etc. All 
ihe latter appear to have possessed the carapace structure 
of Rhinocaris columbina, and if they are to be considered 
as representing different modifications of this type, suffi- 
cient to entitle them to distinct generic appellations, the 
following division of them may be preferable to that now 
standing: under Zhinocaris; R. columbina, Mesothyra veneris ; 
under Mesothyra; M. oceani, Tropidocaris hamiltoniæ, Rhino- 
caris scaphoptera. 
In regard to the unicarinate genus Argas, or Dithyrocaris, it 
may be stated that as described and illustrated by European 
writers, it has not been shown to existin the paleozoic faunas 
of North America. When the structure of the carapace in the 
type species shall become more precisely known, we shall have 
a better basis for the comparison of these fossils with those 
under consideration, but at present we are not justified in 
assuming that they possessed the same structure as the unicari- 
nate Rhinocarids. 
Leaving these points of systematics, we find in Rhinocaris 
and its allies a remarkable carapace structure, an explanation 
of which it is difficult to find among fossil or recent crusta- 
eeans. We may conceive the double suture to have been a 
temporary modification of the Ceratiocarid type, induced by a 
necessity for movement of the lateral parts of the carapace 
after anchylosis has become established along the original 
median hinge. The evidence favors the belief that the valves 
: were capable of motion along the double hinge, and nearly 
. every specimen demonstrates the fact that the parts were sepa- 
rable along these lines after but slight maceration of the tissues. 
No satisfactory elucidation of the origin and morphological. 
‘significance of this structure appears from a study of the 
embryological phases of Nebalia as given by Metschnikoff, 
- Claus and Packard, nor in the development of the Decapods 
- as shown by Sars, S. I. Smith, Brooks, Bumpus and others. It 
is, nevertheless, in the development history of Nebalia and the 
