1893.] Structure of Carapace in Rhinocaris. 799 
Decapods that the key to the taxonomic value of these features 
must be sought. 
The existence of such a structure among the fossils usually 
classed as Phyllocarida, suggests a question as to the latitude 
of this ordinal term, proposed by Packard in 1879, for the liv- 
ing Nebalia and its fossil allies. The hinging, or median divis- 
ion by suture, of the carapace in many of these creatures, has 
been regarded here as among the Phyllopods, as of minor 
importance by most authors who have agreed to associate with 
Nebalia such fossil forms as Ceratiocaris, Echinocaris, etc. Claus 
used the term Leptostraca (also founded upon the structure of 
Nebalia) with a somewhat more restricted aide: 
meaning. In a recent work on the Cera- 4 —— 
tiocaride of Great Britain, the authors, Pro- 
fessor T. Rupert Jones and Dr. Henry F's. 5. 
Woodward, have divided the Phyllocarida Cn 
into two groups (1) those with univalved, and (2) those with 
bivalved carapaces. The greater number of the genera 
, included in the first of these divisions are 
) imperfectly known and quite obscure in their 
structural relations. Such are Discinocaris, 
Spathiocaris, Aptychopsis, Ellipsocaris, Diptero- 
caris, etc., etc. As the living Nebalia is unival- 
Fic. 6. The fat- ved and without hinge, it is in this division 
ce of that one would expect to find the closest 
Nebalia bipes. (Af approach to its structure, and it is, in fact, the 
€— case that none of the so-called Phyllocarida 
approaches Nebalia so closely in the structure of the test as the 
early paleozoic (Cambrian) genus Hymenocaris. In both, the 
telson is represented by a modification of three pairs of caudal 
spines or setze, and both have about the same degree of abdomi- 
nal segmentation, though Nebalia possess a rostrum, while 
Hymenocaris, as far as we now know, is devoid of one. 
Another very early univalved species, not unlike Nebalia, but 
wonderfully similar to the living Phyllopod Apus, is the Pro- 
tocaris marshii Walcott, from the Olenellus-zone of the Cam- 
brian. The figures here introduced, taken from the works of 
"Waleott and Packard, will serve to show this similarity. The 
Hymenocaris 
