12 U.S. NIATIONAL MUSEUM BULLETIN 271 



become incorporated into the cephalic complex; in some talitrids 

 such as Orchestoidea, the lines of fusion of this thoracic segment to 

 the head are still apparent but in most amphipods such external 

 demarcation is obscure. 



In comparison to crustacean groups such as the Isopoda, the 

 body plan of the Gammaridea is conservative. Extreme deviations 

 from the laterally compressed body with enlarged coxal plates occur 

 in a few families in which the body has become dorsally depressed 

 (Corophiidae, Cheluridae, Podoceridae) . Two or more of the urosomal 

 segments have become fused in the Ampeliscidae, Atylidae, Chelu- 

 ridae, and Kuriidae. Coxae have become deformed or enlarged in 

 some Lysianassidae, Astyridae, and Hyperiopsidae. The body is 

 shortened and puliciform in some Haustoriidae and immensely 

 globular in some Lysianassidae. In the Podoceridae the first urosomal 

 segment has become elongated and in the Eophliantidae and Colo- 

 mastigidae the body becomes subcylindrical as in tanaids. 



Major ornaments, of taxonomic value generally at the specific level, 

 include the frequent occurrence of a rostrum, the differentiation of 

 lateral cephalic lobes, the presence of processes on the peduncles of 

 the antennae (especially the Acanthonotozomatidae and Lepechinel- 

 lidae), and the cuspidation of the pleonal epimera. Dorsal ornamenta- 

 tion is most common among cold-water Gammaridea and occurs in the 

 form of teeth and cusps on the pereonites and pleonites. The first 

 urosomite is often ornamented even in genera without other display. 



The absence of eyes is rarely of taxonomic concern other than at 

 the specific level; indeed many sublittoral oculate species are known to 

 have eyeless populations in bathyal depths. 



The condition of the head in the Gammaridea is highly variable and 

 thus useful to the taxonomist, but its morphology is often left unde- 

 scribed. The "basic" gammaridean generally has the head about as 

 long as 1.5 pereonites but it varies in different families and their genera 

 from much shorter than the first pereonite to as long as the first 

 three pereonites combined. The elongate head is especially noticeable 

 in the Ampeliscidae, Phoxocephalidae (including the visor-like 

 rostrum), Synopiidae, and Oedicerotidae. In the latter two families 

 the head is considered "massive" because it is not only elongate but 

 very deep. Other families, such as the Stegocephalidae may have deep 

 heads but they are much shorter than the first three pereonites 

 combined. Recognition of Synopiidae is almost fully dependent on the 

 final confirmation of a "massive" head. Most members of that group 

 have a massive head primarily because pereonites 1-3 are so short 

 that the head appears relatively large by comparison. 



The presence and/or condition of the cephalic rostrum is only 

 occasionally conservative at the familial level (e.g., Phoxocephalidae, 



