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 Arty stone trysihia. 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 

 oestrum in all essential points ; and the same holds good with 

 regard to the antennae and the organs of the mouth. The an- 

 terior corners of the first ring scarcely reach the eyes ; the 

 greatest breadth of the animal is betv/een the fourth and the 

 fifth 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 coxse 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 oestrum. 



This parasite seems to indicate even a more direct transition 

 to the Bopyri than the twisted Livonecm. 



11. In Cymothose 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 



