1254 THE VOYAGE OF H.M.S. CHALLENGER. 



For the original definition of the genus, see Note on Lamarck, 1801 (p. 66) ; Mayer 

 characterises it as follows : — 



" The third and fourth segments of the Perason are without legs. 



" The Mandible is without a palp. 



" There are only two pairs of Branchial Vesicles, these being attached to the third 

 and fourth segments of the perseon. 



"The Lower Antennas have a two-jointed flagellum." 



He adds that the dimorphism, so strongly developed in many species of the genus, 

 to which Kr0yer called attention, is brought about by the circumstance that in the male 

 during growth an enormous elongation of the front part of the body often takes place, 

 sometimes bringing the second gnathopods not only near to the distal end of their own 

 segment, but sometimes to the middle of the animal's body. In this change the females 

 and young males are not concerned. He considers that Haller has pointed out a useful 

 distinction between species which have on the lower antenna? motor-setse (Euder- 

 borsten), and those which have sensitive seta? (Sinnesborsten). Another mode of 

 grouping species he finds in the distinction between those which in the adult male have 

 the first joint of the second gnathopods very long, and those in which it remains short. 

 Of minor and less decisive distinctions, he refers to the length of the upper antenna? and 

 the number of joints that they have in the flagellum, and to the position of the palmar 

 spines on the last three pairs of perseopods. 



Caprella equilibra, Say, 1818. 



1818. Caprella equilibra, Say, Journ. Acad. Nat. Sci. Pliilad., vol. i. 



1843. „ Januarii, 1 Kr0yer, Naturh. Tidsskr., R. 1, Bd. iv. p. 499, tab. vL figs. 14-20. 



1847. „ equilibra, White, List of Crust, in Brit. Mus., p. 92. 



1850. ,, Kroyeri (T), de Haan, Fauna Japonica. 



1852. ,, Januarii, Dana, U.S. Explor. Exped., vol. xiii. pt. ii. p. 819, pi. Iv. fig. 2. 



1860. „ EsmarUi, Boeck, Forh. ved de Skand. Naturf. 8de Mode, p. 674. 



1860. ,, laticornis, Boeck, Forh. ved de Skand. Naturf. 8de Mode, p. 675. 



1862. „ xquilibra, Spence Bate, Brit. Mus. Catal. Amph. Crust., p. 362, pi. lvii. fig. 5. 



1866. „ „ Bate and Westwood, Brit. Sess. Crust., vol. ii. p. 71 (with figure). 



1866. ,, monacantha, Heller, Beitrage zur naberen Kenntniss der Amph. des Adriat. 



Meeres, p. 54, taf. iv. figs. 17-19. 

 1870. „ laticornis, Boeck, Crust, amph. bor. et arct., p. 194 (274). 

 1870. „ Esmarkii, Boeck, Crust, amph. bor. et arct., p. 195 (275). 

 1876. ,, laticornis, Boeck, De Skand. og Arkt. Amph., p. 689, pi. xxxii. fig. 10. 

 1876. „ Esmarhii, Boeck, De Skand. og Arkt. Ampb., p. 693, pi. xxxii. fig. 5. 

 1878. „ equilibra, Gamroth, Naturgeschichte der Caprellen, Zeitscbr. f. wiss. Zool., 



Bd. xxxi. p. 101. 



1 Spence Bate and Mayer both refer for this species to the Voy. en Scand., pi. vi. fig. 15, but there is no species so 

 named on that Plate or in any part of the work referred to. The reference in the Brit. Mus. Catal. Amph. Crust., p. 362, 

 was probably intended for a reference to the Naturh. Tichskrift, which Mayer gives correctly. 



