THE HORSE FOOT CRAB. 261 
fore me. Was it not a veritable trilobite? It at once began 
to shift for itself, making a persistent effort to burrow like its 
parent. By consulting the figure it will be seen that besides 
its tail-less aspect every feature is that of a trilobite. The 
abdominal, or caudal, carapace is relatively much wider than 
in the adult Limulus. The segmentary lines afford a very 
distinct trilobed character to both shields. The spiny and se- 
taceous fringe finds its counterpart in many of the trilobites. 
The pointed tendency of the keel on the caudal shield seems 
to me to look towards Pterygotus. But if we take into view 
the presence of the ocelli already, and the high-up position 
of the large sessile eyes, we have Eurypterus shadowed 
forth. Let the reader examine Pl. 3, figs. 5, 6, 7, 8, 9, 10, 
which give an outline of the telson-plate, or terminal tail- 
joint of as many separate species of the fossil crustacean , 
Pterygotus. Fig. 5 is P. Banksii, in which the telson is 
marked by a cleft. Fig. 6 is P. bilobus, showing the cleft 
less marked, and the presence of a median ridge or keel. 
Fig. 7 is P. gigas, in which the keel is more developed, 
showing a higher relief, and a greater prolongation, and the 
disappearance of the cleft. Fig. 8 is P. Ludensis. Here 
the keel is still more acuminated, and the plate itself is 
mucronated. Fig. 9 is P. bilobus, its size being very much 
reduced. Fig. 10 is P. acuminatus. Here the keel has 
attained an extreme length, and great relief, and is with the 
plate carried to a slender point. And this prolongation of 
the telson plate into a terminal spine, is, I think, in respect 
of posterior development, the highest effort of the Pterygotus. 
I also think that this is shadowed forth in the embryology of 
Limulus. But it should be noticed that there is not so far, in 
all this spinal tendency, anything in the direction of an articu- 
lated spine. That is, there is nothing xiphosuroid, or sword- 
tailed in all this, as in Limulus, and the fossil crustacean 
Eurypterus, which have an articulated bayonet-shaped ap- 
pendage. Now Pterygotus has two sessile eyes, and only 
two, and these are placed low down on the very edges of the 
