1893.] Structure of Carapace in Rhinocaris. 801 
Nebalia except in this division of the carapace; a rostrum was 
present in most, though in some, as Echinocaris, a large amount 
of material has as yet failed to establish its existence. Nebalia 
possesses stalked compound eyes, which make no node or other 
configuration upon the exterior of the carapace. In Echino- 
caris, one of the numerous nodes in the cephalic region is 
undoubtedly ocular, and sometimes shows a slight depression 
at its summit; the other nodes are probably of muscular origin. 
In Ceratiocaris there is no external evidence of eyes, while 
Emmelezoe, Tropidocaris and Elymocaris all possess nodes 
which may be definitely referred to the ocular organ. 
Figures of Emmelezoe given by Jones and Woodward indicate 
that its structure is similiar to that of Rhinocaris, and it is evi- 
dent that differences of structure in this respect between all of 
these genera and Mesothyra as represented in the accompanying 
figures, is simply one of degree. It is, therefore, a pertinent 
query whether such a fixed external ocular body as this, with 
a single central depression, is in any way indicative of stalked 
compound eye. . We are strongly of the opinion that it is not, 
but, rather, indicates that these ancient representatives of 
nebaliad structures were sessile eyed. 
Among the crustaceans we have been considering there 
appear to be at least four types of test-structure which are 
well distinguished. 
1. That of Apus (Protocaris; a synthetic type, not necessa- 
rily a Phyllopod because Apus is one. 
2. That of Nebalia, in a restricted sense (Hymenocaris). 
3. That of Ceratiocaris (Echinocaris, Elymocaris, etc.). 
4. That of Rhinocaris (Mesothyra, Dithyrocaris ? ?). 
It was proposed, when Rhinocaris was believed to represent 
a univalved, rostrate carapace, to distinguish it not only as a 
genus, but as a separate family, RAinocaridz, from other Phyl- 
locarids; and it was at the same time proposed to place Meso- 
thyra, with a supposed structure which we have now shown to 
exist in Rhinocaris, in a distinct family, the Pinnicaridx. It is 
now evident that the two fossils are very closely related and it 
will not do to separate them by more than a generic difference; 
we may therefore retain the family term Rhinocaridx and dis- 
card the other. 51 
