134 The American Naturalist. ` [February, 
In my latest paper ('93, pp. 252-254) I repeated the same ideas, 
and, within a short time of this paper, appeared Mr. Matthew's 
notice (93) of the existence of true Crustacean antenne in 
these forms. This, combined with the truly Crustacean tho- 
racic appendages already described by Wolcott (81 and’ 84), 
and the utter inability, upon careful analysis, to homologise 
the regions in Limulus and the Trilobites, is sufficient to 
divorce the two and to assign the latter to the Crustacea. 
Exactly what position they should occupy here is uncertain. 
It is undeniable that they present a superficial resemblance to 
tlie Isopoda. In both there is the same depressed body, the 
division of this into the three regions of head, thorax and 
abdomen, the head in both cases bearing sessile compound 
eyes; but at this point the resemblance ceases. In the Isopod 
the thorax is always 7-jointed ; in the the Trilobites the num- 
ber varies very considerably. In the Trilobites the append- 
ages, as restored by Walcott, surround the mouth, much as in 
Limulus or the Scorpions; in the Isopods the arrangement 18 
truly Crustacean. In the Isopods, in the embryo, as well as 1D 
the adult, the thoracic appendages consist of but a single 
branch, and when any other structures are present, as for 
instance, the plates forming the brood-pouch, these are placed 
mediad to the insertion of the limb; the gills (?) in the Trilo- 
bites are outside the point of articulation, while the limbs, a$ 
already stated, are dichotomously branched. In the Isopods 
the respiratory organs are lamellar appendicular plates 
beneath the abdomen; we do not know exactly what struc 
tures are found here in the Trilobites. Professor Micklebor 
ough (’83) thinks that there are lamellar plates, but Mr. Wal- 
cott (’84) studying the same specimens believes that the ap- 
pendages of this region resemble those of the thorax.’ So 
it would appear that the Trilobites have no close affinities 
with the Isopoda; the resemblances to the Amphipods ay 
even less close. So far as I am aware, there is no recent Crus 
tacean which presents any resemblance to Trilobites closet 
than those of the Edrophthalmia just discussed, and y et we 
ê The process cuts illustrating Mr. Matihew’s article gives no intelligible details of 
the foot structure in Triarthrus. 
