MUSEUM OF COMPARATIVE ZOOLOGY. 43 



ably beyond the grooves, but when the abdomen ia fully extended the pro- 

 cesses are withdrawn so as to expose the dorsal part of the groove, and in this 

 position in the contracted alcoholic specimens the somites are firmly clamped, 

 apparently by the pressure of the ends of the processes upon the concave pos- 

 terior walls of the grooves, and held rigidly extended, so that it is very difficult 

 to flex the somites, unless the tip of the abdomen is pulled backward with con- 

 siderable force, when the processes slide easily through the grooves and the 

 somites are readily flexed. It is probable that in life, while the extensor muscles 

 of the abdomen are relaxed, the processes move easily through the grooves ; 

 but when the extensor muscles are strongly contracted the hinges are clamped 

 as in the alcoholic specimens, so that the animal can voluntarily hold the telson 

 and the spiny terminal somites of the abdomen rigidly extended as a means of 

 self-defence. 



In all three of the species, when the abdomen is fully flexed, the tip of the 

 telson is brought directly below and very near to the mouth. 



A. Milne-Edwards, in the paper already referred to, which has been pub- 

 lished since the part of this report relating to the Crangonidse was ready for the 

 printer, has described three new species belonging to a new genus, Glypho- 

 crangon, which is apparently very closely allied to the genus here described 

 and possibly identical with it. In Milne-Edwards's genus the telson is de- 

 scribed as consolidated with the sixth somite of the abdomen.* It is scarcely 

 to be supposed that Milne-Edwards could mistake the peculiar articulation of 

 the telson with the sixth somite of the abdomen, which is described above and 

 which is equally characteristic of the articulation of the sixth somite with the 

 fifth and of the fifth with the fourth, for actual consolidation, or overlook the 

 remarkable character of the articulation of the external mazillipeda with the 

 carapax ; and as neither of the species here described and figured agrees fully 

 with the description of either of the species of Glyphocrangon in the Bpinea 

 of the carapax and abdomen, I am forced to the conclusion that Milne- 

 Edwards's genus is different from mine, though possessed of quite as remark- 

 able characters. 



Rhackocaris Agassizii, sp nov. 



Plate V. Fig. 3. Plate VI. Fig. 2. 



Female. — The carapax has eight conspicuous longitudinal carina 1 which are 

 interrupted by a very deep cervical and a broad and deep gastro-orhital sulcus, 

 but, aside from the carinas, sulci, and spines, is Dearly cylindrical The rostrum 

 is about two thirds as long as the vest of the carapax along the dorsal line, tlat- 



* In characterizing the genus he Bays, " Le septieme article abdominal est 

 presque entierement soude* an sixieme"; and in the description of G. tpi*ioauda, the 

 lust species, "Le septieme article est immobile snr le pr£ce*dent, il eat triangulaira, 

 mcarene* en dessus, tres pointu, et nu lien d'avoir la meme direction qne Lei 



articles, il se releve et son extremitl est dirig£e en haut." 



