AMPHIPODA FROM BAY OF BISCAY. 215 



slender than in falcata. The cuticle is thinner, and the coloration 

 also differs, as Sars noted when first describing the species (29), p. 112, 

 In falcata the pigment is in dark definite bands or patches composed 

 of stellate markings or of dots thickly crowded together, retaining its 

 colour even after years in alcohol. The distribution of colour can be 

 plainly seen in Montagu's specimen even now after a century. In 

 pusilla, on the other hand, the pigment shows only as diffused indefi- 

 nite transverse bands along the posterior margins of the peraeon-seg- 

 ments, and occasionally on the pleon-segments as well ; spirit specimens 

 retain very little of the colour. 



Size. The difference in size is given by Sars (29), p. 112, as a specific 

 character, but, as Walker pointed out, it cannot be used as such, for 

 though pusilla is always small, never exceeding 5-6 mm., and falcata 

 attains a length of 10-12 mm., yet specimens of the latter, both male 

 and female, have been found sexually mature at 4 mm. 



Sidcplates (PI. Ill, figs. 10 and 11). The proportions of the sideplates 

 are generally given as specific characters, but they cannot be relied 

 on as such, varying as they do with the age of the animal. 

 Stebbing gives the falcata (41), p. 654, "3rd and 4th in S consider- 

 ably deeper than 2nd and 5th " ; and for pusilla ^ " 5th nearly as 

 deep as 4th." The sideplates in this latter species are always wider 

 and much deeper in proportion to the body than in falcata; Side- 

 plate 2 is rounded anteriorly, considerably expanded inferiorly, and 

 twice as wide as deep (fig. 11); the 3rd and 4th the deepest, as wide 

 as deep; the 5 th almost as long as the 4th. In falcata the relative 

 proportions alter with each stage of development, the only constant 

 feature being the short anterior margin of Sideplate 2. In the young 

 in the marsupium 2'25 mm. long, the inferior margins of Side- 

 plates 1-5 are on the same level ; in specimens 5-6 mm. in length, 

 the 3rd, 4th, and 5th are on the same level, the 1st and 2nd shorter ; 

 at 7 mm. length, the 5th is slightly shorter than the 4th ; while in 

 large specimens 9 mm. and upwards, the 1st, 2nd, and 5th are very 

 noticeably shorter than the 3rd and 4th. 



Antennae. Here again the proportions of the joints of the peduncles, 

 and tlie number of joints in the flagella vary with the growth of the 

 animal, and cannot be employed in specific distinction. The antennae 

 in jij?6si7/« are much more slender, and the primary and accessory 

 riagella much longer in proportion than in falcata. The joints of the 

 primary flagellura are long and cylindrical, and the accessory flagellum 

 is longer in proportion to the 1st joint of the primary than in the 

 other species, equalling half the length of the 1st joint in the adult 

 animal. Tlie last joint of the peduncle of the inferior antenna is fringed 



