THE HORSE FOOT CRAB. 273 
when it walked. While tailless there were simply two par- 
allel sets or rows of toe-tracks, but when tailed those 
parallel rows were separated by a median line, showing the 
caudal trail. Is there not here a caution for the interpreters 
of the “Protichnites” seeing that the same species, at diverse 
ages, may make widely different tracks? 
Not more than three or four exuvie were found entire in 
the mud of the hatching jars. In all the rest the buckler 
and the pygidium were separate. Now it is not the case 
that the Horse Foot shells, in the long wind-rows on the 
shore-line, are entire. The entire ones are decidedly excep- 
tional. Before certain tides the young are helpless; but the 
adult never comes shoreward except to spawn. Hence their 
exuvie are brought up by the wash and the under-tow of 
storms, thus effecting the separation of the two parts. Is 
there not here an explanation of the great abundance of the- 
pygidia, or caudal shields, of the Asaphus Iowensis in the 
Iowa limestone rocks? I do not regard them as the debris 
of dead trilobites but as their cast-off shells. They are the 
tidal windrows of that ancient sea. The articulation of the 
two carapaces was no doubt feeble; and the specific gravity 
of the pygidium less than that of the buckler. In this case 
the debris would be sorted into different depths of water. 
The bucklers would be less crowded, because in greater 
depths where the tidal action was less; while the lighter 
pygidia would, by the same law, form the drift of the shore- 
lines. 
EXPLANATION OF PLATE 3. 
Fig. 1. Egg of Limulus just cracked by incubation, showing the pellucid 
sphere. f 
Embryo in the egg, much enlarged. 
Same two days older, much enlarged. 
Young Limulus just out of the egg, enlarged nine diameters. 
Terminal tail joint of Pterygotus Banksii. 
Terminal tail joint of Pterygotus bilobus. 
Terminal tail joint of Pterygotus gigas. 
Terminal tail joint of Pterygotus ludensis. 
AMER. NATURALIST, VOL. IV. 
ie 
BON NONO 
SAPS wh 
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