114 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
In three of the genera of the Nebaliidas, the eight pairs of thoracic limbs are all simi- 
lar to one another, though those of the genera differ. All are biramous. The limbs of 
Hymenocaris can apparently be most closely correlated with those of Nebalia antarctica, in 
which the endopodite consists of short flattened segments, and the exopodite is a long seti- 
ferous plate. Epipodites are present in both Nebalia and Hymenocaris. 
So far as the appendages of Hymenocaris are known, they agree very well with those 
of the Nebaliidas, and since they are of the trilobite type, it may safely be stated that the 
Trilobita and Malacostraca are closely related. 
2. Syncarida. 
Walcott (1918, p. 170) has compared the limbs of Neolenus with those of the syn- 
carid genera Anaspides and Koonunga. These are primitive Malacostraca without a cara- 
pace, but as they have a compressed test and Anaspides has stalked eyes, their gross anatomy 
does not suggest the trilobite. The thoracic appendages are very trilobite-like, since the 
endopodite has six segments (in Anaspides) and a multisegmented setiferous exopodite. 
The coxopodites, except of the first thoracic segment, do not, however, show endobases, and 
those which are present are peculiar articulated ones. The cephalic appendages are special- 
ized, and the antennules double as in most of the Malacostraca. External epipodites are 
very numerous on the anterior limbs. 
This group extends back as far as the Pennsylvanian and had then probably already 
become adapted to fresh-water life. It may be significant that the Palaeozoic syncarids 
appear to have lacked epipodites. While differing very considerably from the Trilobita, 
the Syncarida could have been derived from them. 
3. Isopoda. 
Since the earliest times there has been a constant temptation to compare the depressed 
shields of the trilobites with the similar ones of isopods. Indeed, when Scrolls with its 
Lichadian body was first discovered about a hundred years ago, it was thought that living 
trilobites had been found at last. The trilobate body, cephalic shield, sessile eyes, abdom- 
inal shield, and pleural extensions make a wonderful parallel. This similarity is, however, 
somewhat superficial. The appendages are very definitely segregated in groups on the vari- 
ous regions of the body, and while the pleopods are biramous, the thoracic legs are with- 
out .exopodites (except in very early stages of development of one genus). The Isopoda 
arose just at the time of the disappearance of the Trilobita, and there seems a possibility 
of a direct derivation of the one group from the other. It should be pointed out that while 
the differences of Isopoda from Trilobita are important, they are all of a kind which could 
have been produced by the development from a trilobite-like stock. For example : 
Isopoda have a definite number of segments. There is less variation in the number 
of segments among the later than the earlier trilobites. 
Isopoda have no facial suture. In at least three genera of trilobites the cheeks become 
fused to the cranidium and the sutures obliterated. 
Isopoda have one or two segments of the thorax annexed to the head. While this 
is not known to occur in trilobites, it is possible that it did. 
Most Isopoda have a fairly stiff ventral test. The ventral membrane of trilobites 
would probably have become stiffened by impregnation of lime if the habit of enrollment 
had been given up. 
