OF THE AMERICAN KING-CRAB. 495 
following remarks bearing upon the correlation of vision with other ways and means of 
locomotion. 
Limulus possesses the pair of relatively large compound eyes, set high upon the 
lateral parts of the cephaletral carapace; and besides these, it has the pair of small 
anterior simple eyes: it looks forward and upward, and commands, like the guns of a 
demilune bastion, a like range in the horizontal sweep. And yet the prevalent impres- 
sion, from the position and proportions of its subcylindrical jointed limbs, is that they 
subserve the needs of digestion much more than those of locomotion. No observer has 
yet testified to their capacity of uplifting the body from the ground, whether dry or 
submerged, and of bearing it along by successive steps, as do the jointed legs of the 
Isopod, the Lobster, or the Crab. Some of the pairs are obviously incapable of such 
locomotive functions. The last pair (vir. in all the figures) may help to push the body 
along the sand, as the oar serves to shove off a boat; but that is all. 
Eurypterus and Pterygotus possess, like Limulus, both the antero-median ocelli and 
the medio-lateral compound eyes. In P/erygotus the antennæ are forcipated members 
for prehension of food, as in Limulus. The three succeeding pairs of cephaletral limbs 
are still less capacitated, through their proportionally smaller size and concomitant 
slenderness, for gradatorial movement of the body. They are adapted to rout out of 
the sand or mud, disturbed by the spade-shaped head, the objects of food which the 
front pair is modified to seize. The larger terminal pair of limbs are more decidedly 
natatory in form than are the last cephaletral lamelligerous pair (vir) in Limulus. In 
Eurypterus the cephaletral limbs anterior to the lamelliform natatory pair seem to be 
alike in structure, unless the antennal forceps has been wanting in the fossils, and must 
have had functions as limited as are their size and strength. 
These considerations weigh with me in checking a tendency to conclude that the 
Trilobites, because they had large compound eyes, must have had articulate ambulatory 
limbs of as strong a texture, whether crustaceous or chitinous, as their body-segments. 
That sixteen slender freely movable filamentary limbs, as restored by Mr. Woodward*, each 
nearly 13 inch in length, attached by a flexible joint no bigger than a pin's head, and 
divided into seven movable segments by six other joints, in a Crustacean that may have 
undergone, to say the least, some disturbance between death and fossilization—that the 
eight pairs of such articular appendages should remain and be found symmetrically and 
regularly arranged across the ventral surface of the fossil, with intervals, if not parallel to, 
yet corresponding in length with those of the thoracetral segments—presents itself to my 
mind as much less probable than that the narrow parallel ridges which constitute 
the observable phenomena should have had such extent of attachment to the ventral 
surface of the several segments as to offer the requisite physical resistance to displace- 
ment and to loss of original regularity and symmetry of position, such as the specimen 
of Asaphus platycephalus described and figured by Billing t actually presents to view. 
If this Trilobite possessed the ambulatory legs ascribed to it, it could hardly be an excep- 
tion, in this endowment, to its order, and traces of such limbs, in divers conditions of 
displacement, would be common. 
* Geol. Mag. viii. 1871, pl. viii. fig. t a. : + Lo. cit. 
