6 U.S. NATIONAL MUSEUM BULLETIN 2 78 



rately. Unfortunately, it has been one of the most important charac- 

 ters and the correct generic determination usually depends upon 

 elucidating its structure. Mayer stressed the importance of the 

 abdomen by separating closely related genera such as Deutella and 

 Luconacia primarily by differences in the abdomen. Mayer's emphasis 

 on this character is justified; however, due to its vestigial nature it 

 suffers from the same criticism as pereopods 3-5. In deaUng with the 

 many stages of reduction of the appendages on the abdomen, Mayer 

 was inconsistent in what he considered to be a "Klappe" or vanished 

 appendage. This is especially true in those genera which do not 

 bear true appendages but which have several setae or even a single 

 seta borne on a type of flap or lobe. For an example of this, compare 

 Mayer's (1903) figures of the abdomen of the Triliropus male (pi. 9 

 fig. 70), which he says bears one-half pair of appendages, with that 

 of the Pseudoproto male (pi. 9 fig. 52) which he claims to be without 

 appendages. Both abdomens have lobes with several setae; therefore, 

 due to this inconsistency I have refrained from using Mayer's termi- 

 nology of one-half appendage pairs but have instead given the number 

 of recognizable appendages and have described the lobes. 



Illustrations and Measurements 



Illustrations of the whole mounts were made by the use of a micro- 

 projector and those of dissected appendages with a camera lucida. 

 Pencil sketches were first made which were later copied on Ethulon 

 tracing film. All scales on the figm'es equal 1 mm for the whole mount. 



Measurements of the total length refer to the length of a fine 

 drawn from the anterior portion of the cephalon between the insertions 

 of antennae 1 and 2, through the midlateral portion of each pereonite, 

 to the posterior tip of the abdomen. 



Key to the Caprellidae of the Western North Atlantic 



(See figure 1 for explanation of characters.) 



1. Mandible with palp or setae representing vestige of palp 2 



Mandible without palp 17 



2.(1) Pereopods 3 and 4 absent 3 



Pereopods 3 and 4 present 6 



3.(2) Abdomen with only pair of lobes . . Pseudaeginella antiguae (p. 100) 



Abdomen with appendages 4 



4.(3) Abdomen with pair of appendages and pair of lobes. 



Aeginella spinosa (P* S) 



Abdomen with 2 pairs of appendages 5 



5.(4) Abdomen with only 2 pairs of appendages. 



Proaeglnina norvegica (p. 97) 

 Abdomen with 2 pairs of appendages and pair of lobes. 



Aeginina longicornis (p. 13) 



