60 U.S. NATIONAL MUSEUM BULLETIN 292 



forming variable, mostly bilaterally symmetrical, patterns along 

 lateral margins of dorsomedian stripe; with increasing age, patterns 

 coalescing and infringing on stripe to extent that in individuals of 

 intennediate size, stripe usually narrower, irregular, and interrupted, 

 and largest individuals usually without trace of stripe. 



Green Phase: Pattern essentially identical; color, however, ranges 

 from pale bluish green to greenish black. 



Material examined. — The Dominican collections contain 232 

 males (carapace lengths 5.0-33.7 mm), 246 females (cl 5.9-20.6 mm), 

 including 80 with eggs (cl 8.8-20.6 mm), and 215 juveniles (cl 

 1.1-5.0 mm). The smallest specimens for which the sex can be deter- 

 mined have a carapace length of about 5.0 mm; at this size, and 

 occasionally at a carapace length of as much as 6.2 mm, the appendix 

 masculina on the second pleopod of males is subequal in length to the 

 appendix interna. 



Ecological notes. — Atya innocous is probably the least ecologically 

 and geographically restricted shrimp on the Island of Dominica. It 

 has been found in such diverse habitats as the mouth of the Layou 

 River, some 100 feet from the Caribbean, and in a small tributary 

 of the Rosalie River between Boeri and Freshwater Lakes at an altitude 

 of approximately 2,500 feet. It seems to be equally at home in the 

 cascading reaches of mountain rivulets, in quiet upland pools, and 

 in low-lying sluggish brooks. In an upland pool, on a stream tributary 

 of the Layou River flomng through the Cassada Gardens Estate 

 (station 29), some 20 or 30 individuals were observed actively crawling 

 over the bottom at about 10:00 a.m. Sharing the pool with them were 

 some 10 or 12 Macrohrachium crenulatum. Here, a large boulder 

 projecting from the bank over the pool provided considerable shade. 

 In their wanderings, both shrimps ventured into areas of direct 

 sunlight but quickly returned to the shaded portion of the pool. In 

 the edge of a riffle area just below the pool, several individuals of 

 M. heterochirus occasionally emerged from beneath stones, but no 

 Atya were observed except in the pool. 



Along Mannet's Gutter, near Clarke Hall, A. innocous was equally 

 common in the riffle areas, in the cascading reaches, and in the pools. 

 (See the description of the decapod composition of a Dominica 

 pool, p. 43.) 



Distribution. — Nicaragua to Panama and the West Indies (Cuba, 

 Jamaica, Hispaniola, Puerto Rico, Saint Croix, Montserrat, Guade- 

 loupe, Dominica, Martinique, Saint Vincent), 



Dominica Stations: 1-5, 7, 8, 13-15, 17-21, 26-32, 34, 35, 39, 41, 

 45, 46, 48, 5.3-55, 59, 60, 63, 66, 69, 70, 73-76, 78, 80, 92, 101, 103, 

 105, 109 (0-3,050 ft.). 



