1893.] Structure of Carapace in Rhinocaris. 797 
Rhinocaris, This presumptive structure was exhibited in a 
diagrammatic figure upon Plate xxxii, of which a reduced 
copy is here given. Though neither the rostrum nor the 
median plate in Mesothyra has yet been seen, all 
doubt of their existence is removed, and it affords 
a personal gratification to find this reconstruction 
of Mesothyra so fully substantiated by our present 
knowledge of the carapace in Rhinocaris. 
The structure of the visual organ affords a 
further correspondence in the two genera, and 
one of some morphological significance. 
In regard to the number of abdominal segments, 
present evidence indicates a difference in the two, 
Rhinocaris columbina having at least three, and 
Mesothyra having shown but two. 
In the genus Tropidocaris Beecher, it has been 
shown that the type species, T. bicarinata, possesses 
a rostrum, and its anterior extension is not unlike 
that of Rhinocaris, but there is now no satisfactory 
evidence of a free median plate in any of these 
Fic. 4. The Species. Unfortunately the accessible specimens 
median plate of Of the species are not very satisfactorily pre- 
Rhinocaris col- served, and the originals described by Mr. 
anise Beecher in Report PP of the Second Geological 
Survey of Pennsylvania, are understood to be beyond the reach 
of further investigation for the present. In plaster casts of the 
originals, the median suture on the hinge appears to be invaria- 
bly in the longitudinal axis of the test. The original specimens 
of Rhinocaris scaphoptera and Tropidocaris hamiltoniæ, which 
were collected by me in the Hamilton shales of Ontario County, 
N. Y., both show some evidence of a quite narrow median plate, 
and the former possesses a stout rostrum,* while in the Meso- 
thyra (Dithyrocaris ?) veneris, from the Marcellus shales, both 
median plate and rostrum are distinctly seen.° 
It may be questioned if the multicarinate carapaces com- 
posing the genus TJropidocaris should be placed in close asso- 
1.05. cit. pl. xxxi, fig. 22, 29. 
5 Of. cit. pl. xxxiii, fig. 3. 
