PROSOMATIC SEGMENTS OF LIMULUS 249 
of segmentation in this region afforded by the examination of the 
invertebrate, whether living or fossil, so as to see what clues are left 
if the evidence of appendages fails us. I will take in the first instance 
the evidence of segmentation afforded by the presence of the muscu- 
lature of Group 4, even when, as in the case of many fossils, no 
appendages have yet been found. In such animals as Mygale and 
Phrynus the prosomatic carapace is seen to be marked out into a 
series of elevations and depressions,and upon removing the carapace 
we see th@t these elevations correspond with and are due to the large 
tergo-coxal muscles of the appendages ; so that if such carapace alone 
were found fossilized we could say with certainty: this animal pos- 
sessed prosomatic appendages the number of which can be guessed 
with more or less certainty by these indications of segments on the 
carapace. 
In those forms, then, which are only known to us in the fossil 
condition, in which no prosomatic appendages have been found, but 
which possess, more or less clearly, radial markings on the prosomatic 
carapace resembling those of Phrynus or Mygale, such radial markings 
may be interpreted as due to the presence of prosomatic appendages, 
which are either entirely concealed by the prosomatic carapace or 
dorsal head-plate, or were of such a nature as not to have been 
capable of fossilization. 
The group of animals in question forms the great group of animals, 
chiefly extinct, classified by H. Woodward under the order of Mero- 
stomata. They are divided by him into the sub-order of Eurypteride, 
which includes—(1) Pterygotus, (2) Slimonia, (3) Stylonurus, (4) 
Eurypterus, (5) Adelophthalmus, (6) Bunodes, (7) Arthropleura, (8) 
Hemiaspis, (9) Exapinurus, (10) Pseudoniscus; and the sub-order 
Xiphosura, which includes—(1) Belinurus, (2) Prestwichia, (3) 
Limulus. | 
The evidence of the Xiphosura and of the Hemiaspide conclusively 
shows, in Woodward’s opinion, that the Merostomata are closely 
related to the Trilobita, and the Hemiaspide especially are supposed 
to be intermediate between the trilobites and the king-crabs. They 
are characterized, as also Belinurus and Prestwichia, by the absence 
of any prosomatic appendages, so that in these cases, as is seen in 
Fig. 12 (p. 30), representing Bunodes lunula, found in the Eurypterus 
layer at Rootzikill, we have an animal somewhat resembling Limulus 
in which the prosomatic appendages have either dwindled away and are 
