MYSIDEA. 667 



RACHITIA SPINALIS. 



Carapax antice acute" et longe tricuspidaius, spind medid (vel rostro) 

 longiore (oculis duplo long lore), lateralibus ascendentibus, posticd 

 angustus, transversus, marginibus lateralibus integris. Segmenta ab- 

 dominis quinque spind longd acutd dorso armata ; segmentum scxtum 

 segmentis quatuor precedentibus simul sumtis fere longim. 



Carapax long and acute tricuspidate in front, middle spine (or beak) 

 longest (twice as long as the eyes) , lateral flexed upward ; poste- 

 riorly narrow and margin transverse, lateral margins entire. Five 

 abdominal segments having a long acute spine on the back, sixth 

 segment hardly shorter than the four preceding together. 



Plate 44, fig. 8 a, b, different views of animal, enlarged ; c, caudal 

 extremity. 



Atlantic, off the harbour of Rio de Janeiro. Collected, January 7, 

 1838. 



Length, one-eighth of an inch. Colourless. The beak is straight 

 and horizontal, while the spines either side are bent upward, as seen 

 in a lateral view. The carapax, in a vertical view, is widest just 

 anterior to middle, then narrows regularly till it becomes as narrow 

 as abdomen at its posterior part. The first five abdominal segments 

 are not oblong, and the dorsal spine is rather longer than the segment; 

 the spine is directed backward and upward. These five segments are 

 about as long as carapax, the last slightly longer than preceding. The 

 sixth segment is nearly as long as preceding five. In the movements 

 of the animal, the body is often flexed at the articulation preceding 

 this segment. There is a spine on the under side of fifth segment, 

 near the articulation with the sixth segment. The last segment of 

 the body is somewhat triangular, narrowest at base, and terminates in 

 two rounded lobes, each of which is nearly as broad as the segment 

 at base ; the sinus between the lobes is rounded. Three or four seise 

 at the extremity of each lobe are as long as the segment ; besides 

 these there are two or three shorter on inner side, and one on outer 

 margin of lobe. 



