the Mouth in Sucking Crustacea. “13 
in the Plata River, but of which the specific name was not 
determined. As this Isopod is apparently undescribed, and 
rather remarkable, I here insert its chief characteristics. 
I propose to call this species Artystone trysibia. It re- 
minds one somewhat both of Urozeuctes and of Olencira, but 
is easily distinguished from the other genera of the family by 
the striking contrast between the first six pairs of legs, which 
are short and hooked, and the seventh pair, which reach to 
the extremity of the tail and are slender, compressed; crawl- 
ing legs with small, almost rudimentary, straight claw. The 
trunk is slightly vaulted, broadly elliptic, the fore part some- 
what twisted to the left, the hind part twisted still more to 
the right. The head is small, resembling that of Cymothoa 
. estrum in all essential points; and the same holds good with 
regard to the antennz and the organs of the mouth. The an- 
terior corners of the first ring scarcely reach the eyes; the 
eatest breadth of the animal is between the fourth and the 
th ring, where it is about half the greatest length. All the 
rings of the trunk have a couple of small, triangular, irregular, 
lateral folds over the epimera, which latter are luniform, 
rounded before and behind, extending on the first four rings 
as far as about one-half of the lateral margin of the dorsal 
plate, on the fifth reaching almost the whole length of the 
margin, and on the sixth and seventh somewhat beyond the 
dorsal shield. The coxz are even, without protuberance, the 
hooked legs small, almost of equal length, their claws very 
fine. The first five rings of the tail are very short, of uni- 
form length, somewhat increasing in breadth behind, the first 
three covered by the seventh ring of the trunk ; the last caudal 
ring is gradually narrowed from the base, obliquely triangular, 
of about equal length and breadth, rather high-arched, with 
rounded apex; the last pair of caudal legs reach not quite to 
the apex of the ring ; the branches are of equal size, elongated 
elliptic, soft, naked. The total length is 13 millims. The 
colour is white; on the trunk very minute points of black 
pigment are observable on the side folds of the dorsal shields, 
on the last three pairs of epimera, and across the dorsal shields 
along their posterior margin; vestiges of similar points are 
seen in a streak along the tail and on the base of the last joint. 
The only specimen found is a female with ripe eggs; the 
opercula are as in Cymothoa estrum. 
This parasite seems to indicate even a more direct transition 
to the Bopyri than the twisted Livonece. 
11. In Cymothoe the organs of the mouth are adapted for 
sucking in the following manner. The labrum is elongated, 
transversely curved so as to form a semicylindrical duct, which 
