14 University of California Publications in Zoology (Vou. 28 
both or one only. When chelate or subchelate they are 
known as chelipeds. In the Brachyura, or true crabs, 
they are always chelate. When simple they are more 
properly called ambulatory legs. 
10, 11, 12 and 18. The next four pairs of thoracic appendages 
are the second, third, fourth, and fifth pairs of legs. 
They may be either simple, chelate or subchelate, all or 
in part only. Although usually spoken of as the first, 
second, third and fourth pairs of ambulatory legs (ex- 
cept when the first legs, no. 9 above, are simple and 
included in the total number of ambulatory legs), 
strictly speaking only the simple pairs are ambulatory 
in the true sense of the word. In the Brachyura these 
four pairs of appendages are frequently designated 
simply as the first, second, third or fourth pairs of legs, 
in contradistinction to the chelipeds (no. 9, above). 
This nomenclature, however, has not been adopted in the 
characterizations of the species listed in this paper. 
All mention of gills and branchial formulae has been 
purposely omitted as they are more or less difficult for the 
beginner or average student; only for the specialist have they 
peculiar interest. 
III. The Abdomen, composed of six true somites, or segments, and a 
terminal joint, or segment, the telson, a flattened usually 
pointed tail-piece which never bears appendages and is not 
considered a true somite. 
14, 15, 16,17, and 18. The first five pairs of abdominal appen- 
9? 
dages are the pleopods, or ‘‘swimmerets,’’ as they are 
sometimes called. The first one or two pairs in the males 
of some.forms are variously modified to serve as acces- 
sory sexual organs; one or more, or all of the pleopods 
may be reduced, rudimentary, or wanting. 
19. The last pair of abdominal appendages are the uropods 
and when present they constitute together with the tel- 
son the so-called ‘‘tail-fan.”’ 
The following seven diagrammatic figures give a graphic exposition 
of the parts, appendages, and body regions of typical decapod crus- 
taceans : 
