GAMMARIDEA. 905 



in the genus Pyctilus (Dana), — a genus closely resembling Erich- 

 thonius and possibly the same, — and also in Dercothoe (Dana). 



3. One-branched, the branch naked and subulate, without spines at 

 apex, or ending in a single spine, appearing as the continuation of the 

 branch itself, and hardly as an added spine. Found in the genus 

 Stenothoe (Dana). 



4. One or two-branched, the inner branch nearly or quite obsolete; the 

 other long, often very long and far exsert, furnished with short hairs, 

 and unlike the branches of the first and second pairs of stylets. The 

 long branch may be either cylindrical or lamellar, and when the 

 latter, the edges may be ciliated. This form occurs in certain species 

 of the genera Amphithoe and Gammarus. 



5. Two equal or subequal branches, which are straight, either short 

 or long, and end in short hairs, or more rarely with spines, these hairs 

 or spines not reflexed like spines at the apex of the branches in the 

 first and second pairs. A prominent division of Gammarus and also 

 one of Amphithoe are thus characterized. 



6. Two equal or subequal branches, the branches dissimilar and quite 

 short ; one of them, the outer, subconical and biuncinate at apex, the 

 two reflexed hooks in the same longitudinal line; the other com- 

 pressed, with a truncate extremity, and having a few minute hairs at 

 apex. A prominent part of the genus Amphithoe has stylets of this 

 kind ; the form is not known to occur among the Gammari. 



7. Two subequal branches, the branches similar, subulate, nearly 

 straight, ending in a slender point, like the form in No. 3. Occurs in 

 the genus Leucothoe. 



The epimerals exhibit also wide variations, corresponding in part 

 with the distinctions afforded by the stylets. These variations con- 

 sist in the relative sizes of the fourth and fifth epimerals, and the form 

 of the fifth. 



In one large division of the genus Amphithoe (the same that is 

 characterized by the sixth kind of posterior stylets), the fifth epimeral 

 is even larger than the fourth, or, at least, not smaller, and instead of 

 being subequally two-lobed, the posterior lobe is very small, and is on 

 the posterior margin of the epimeral. 



In other species of Amphithoe, characterized by posterior caudal 

 stylets of a different kind, the fifth epimeral is much smaller than the 

 fourth, and subequally two-lobed. 



227 



