344 Remarks on the Genus Paradoxides of Brongniart. 
The peculiar organization of the tail naturally divides the genus 
into two sections, and all the species, as far as they are known, may 
be arranged as follows. 
Genus TRIARTHRUS. 
Tail simple, or without membranaceous expansion. 
T. Becki. 
T. Gibbosus. (P. Gibbosus of Brong.) 
! Tail with membranaceous expansion. 
T. Scaraboides. (P. Scaraboides, Brong.) 
T. Laciniatus? (P. Laciniatus, Brong.) 
In the last number of your Journal there is a very interesting 
paper on some trilobites, by James Hall, Esq., and he has had the 
good fortune to discover and describe the first perfect specimen of 
our Triarthrus Beckii, the only American species of this genus 
which I suppose has yet been found. His excellent paper contains 
the figure and description of what he imagines to be another species, 
but judging from the representation he has given of it, and from the 
numerous heads or bucklers of the animal in my possession, the cur- 
vature of the external margin of the cheeks, which seems to consti- 
tute the principal peculiarity, has been produced by the manner in 
which the animal has been fossilized. 
Before closing these remarks, 1 may mention an objection which 
has been urged against the genus ‘Triarthrus, because it was founded 
on the supposition that the buckler was the abdomen of the animal. 
This objection, I think, will not be considered valid, especially as the 
term Triarthrus, which, though originally applied to the three divis- 
ions or articulations, of what was then supposed to be the abdomen, 
may now, without much latitude of interpretation, be considered as 
referring to the three lobes, or longitudinal divisions of the body, the 
name would then be analogous to Trilobite, Trimerus, and other ap- 
pellations common in zoological nomenclature. At any rate, I now 
propose the name Triarthrus, for the group as above characterized. 
