204 AMPniPODA NOKMALIA. 



( iannnarus, Rathhc, Fauna drr Krtpn, p. 372, and Nov. Act. Loop. 1843. 

 White, Hid. lirit. Cnist. n. i8L>. 

 (itiNsr, Mar. Zoo/, i. p. 141. 



Spcnrc liatr, Si/nap.sis, cVr., Ann. Nat. Hist. Feb. 1857. 

 Liljrhorq ( Division + 1. h.), Of vers, af Konql. Vet. Akad. Forhandl. 



18o4," p. 446. 

 BruzvUus (Division aa—b), Skamh Amph. Gamm. p. 50. 



Slender, lateruUy compi-c.s.secl. Cephalon not produced into a rostram. 

 Pereion and pleon subequal in length. Three po.stenor segments 

 of the pleon having each two or more fasciculi of short stiff sjiincs. 

 . Eyes reniform, oval or linear. Antenna? long, slender, filiform, 

 having the peduncle siibequal wdth the peduncle of the inferior, 

 and carrpng a secondaiy appendage. Mandibles ha^'ing an ap- 

 pendage. MaxUlipcds having a scjuamiform plate, arising from 

 the bases and ischium. Gnathopoda subequal, not largely de- 

 veloped. Pereiopoda subequal ; coxae of the three posterior pairs 

 much shorter than those of the anterior. Posterior paii' of plco- 

 poda biramous. Telson double. 



The genus is here confined to Gammarus proper. It is both more 

 convenient and more natural to place those species in distinct genera 

 which have been arranged by authors under separate divisions of the 

 genus. 



1. Gammarus semicarinatus, n. s. (Plate XXXVI. fig. 3.) 



Last segment of the pereion and first three of the pleon carinated and 

 posteriorly produced to a sharp tooth. Fom-th and fifth segments 

 of the pleon carinated and produced to a tubercle which is crowned 

 with a fasciculus of spines, on each side of which are the fasciculi 

 which are common to the genus. Eyes subreniform. Superior 

 antennae half the length of the animal ; peduncle short, first joint 

 little more than half the length of the cephalon, second shorter 

 than the fii'st, tliird shorter than the second ; flagellum three 

 times as long as the peduncle, having the articuli much longer 

 than broad ; secondary appendage having but two articuli. In- 

 ferior antennse half the length of the superior ; peduncle longer 

 than the peduncle of the superior. Gnathopoda subequal : first 

 pair having the propodos long-ovate, tapering, palm not defined, 

 ' armed with three or four short obtuse spines and a few hairs ; 

 dactylos arcuate : second pair having the propodos ovate, palm 

 slightly oblique, bilobed, armed with three or four short obtuse 

 sijines and a few hairs. Pereiopoda subequal. Posterior pair of 

 pleopoda having the rami unequal, the outer being a little longer 

 than the inner. 



Length -^ths of an inch. 



