REPORT ON THE AMPHIPODA. 1357 



Phronima sedentaria (Forskal) (PI. CLXII., B.). 



1775. Cancer sedentarius, Forskal, Descr. Anim. quae in itin. orient, observavit, p. 95. 



1776. „ „ Forskal, Icones rerum nat. quas in itin. orient, depingi curavit., 



tab. xli. fig. D, d. 

 1796. Cancer (Gammarellus) sedentarius, Herbst, Naturgesch. der Krabben und Krebse, Bd. ii. 



p. 136, tab. xxxvi. fig. 8. 

 1802. Gammarus sedentarius, Scbousboe, Skrivter af Naturkist.-Selskabet, Bd. v. Hfte. 2. 

 1802. Phronima sedentarius, 1 Latreille, Hist. Nat. des Crust, et des Insectes, vol. iii. 



A specimen, which seems to agree with this species as well as any in the collection, 

 is figured on the Plate of the natural size. An enlarged figure of the third perseopod 

 is given for comparison with one drawn to the same scale of that peraeopod in Phronima 

 megalodous. The third perseopod of a young one taken along with the large specimen 

 is also given, drawn to the same scale, and a figure of the terminal portion of the 

 same peraeopod much more enlarged. 



Length, from the front of the head to the apex of the third pleon-segment, an inch 

 and a quarter ; the full length quite an inch and a half. 



Locality. — Station 232, May 12, 1875 ; the Hyalonema-ground, Japan ; lat. 35° 11' 

 N., long. 139° 28' E.; depth, 345 fathoms; bottom temperature, 41°'l ; surface tempera- 

 ture, 64 0, 2. One specimen, female, with young. 



Remarks. — From the very extended distribution of the genus Phronima there 

 arises a probability that it may include several species, but to establish clear marks 

 of discrimination between the species is likely to require very extended research. 

 Though it is easy to distinguish the adult males from the adult females, there are 

 stages of growth when the two sexes are closely alike, and it is quite possible that some 

 species when full grown present a close resemblance to the earlier stages in other 

 species. The available marks depend to a great extent on the lengthening and 

 shortening, the sharpening or rounding, of this apex or of that, on the question 

 whether one tooth is more or less distant from another, or whether a margin is 

 denticulate or crenulate. But all these marks are liable to so much variation, whether 

 dependent (as may be the case) on the individual, or (as is certainly the case) on age 

 and sex, that determinations of species are of necessity very problematical. Even if 

 the limits of variation within any one species were definitely known, it is quite 

 possible that in some of the stages it might be practically indistinguishable 

 from some stage of a different species. In the young ones a tenth of an inch long, from 

 the specimen taken south of Australia, March 9 and 10, 1874, the dactyloptera of the 

 gnathopods were found to be very short, and the broad fourth joint of the third 

 perseopods scarcely longer than the distal width, armed only with a minute front tooth; 



1 Most of the references given in the synonymy of the genus Phronima have to do with Phronima sedentaria, 

 Latreille, so that it is scarcely worth while to repeat the list. 



