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 that 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 Eurypteridse, 

 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) 

 hamulus. 



The evidence of the Xiphosura and of the Hemiaspidaj conclusively 

 shows, in Woodward's opinion, that the Merostomata are closely 

 related to the Trilobita, and the Hemiaspida? 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 Pootzikull, we have an animal somewhat resembling Limulus 

 in which the prosomatic appendages have either dwindled away and arc 



