4 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL.18 



Taxonomic Features of the Genus Ampelisca 



Of primary importance in separating species of the genus Ampelisca 

 are the following characters: 



1. The shape of pleon segment 3. The posterior edge of this segment 

 takes many forms and, when combined with several other features, 

 offers an excellent character for the separation of groups of species. It 

 is not advisable to use the shape of this segment alone as a specific cri- 

 terion because of its marked variability in some species, such as Am- 

 pelisca cristata Holmes. 



2. Dorsal posterior process of pleon segment 4. Two main categories 

 may be separated here: (a) those species in which a lamellar crest is 

 elevated above the segment; (b) those species in which a hump or boss 

 is produced and variously ornamented. A negative value is seen in species 

 such as Ampelisca lobata Holmes, in which the dorsal surface of this 

 segment is only slightly convex. The various shapes assumed by the crests 

 and humps are better shown by accurate illustrations than by descriptive 

 phrases. 



3. Shape of head. The length of the head in relation to the body 

 segments may have some systematic value, but the shape of the rostral 

 area and the lower front margin are of greater importance. The lower 

 front margin of the head may be subparallel to the upper margin, making 

 the head appear elongated and produced. When the margin is oblique, 

 the head appears blunt. The oblique margin may be convex, straight, 

 or slightly concave. 



4. Eyes. Some species lack corneal lenses entirely; other species pos- 

 sess 2 pairs of lenses, of which the lower pair shows the greatest varia- 

 tion in placement. In some cases, the lower pair may be covered by the 

 lower front margin as in Ampelisca hemicryptops K. H. Barnard 

 (1930). Other variations include the placement of the lower lenses 

 either at the lateral angle of the head or below the lateral angle on the 

 oblique margin. 



5. Length of uropod 1. This appendage is of great value in quickly 

 separating species which superficially appear to be similar (contrast 

 Ampelisca mexicana, new species, and A. gnathia, new species). The 

 shortening of uropod 1 and the presence of a stout peduncle may be 

 accompanied by a foreshortening of the urosome. 



6. Peraeopod 5. This appendage is quite variable in the genus 

 Ampelisca and offers many excellent taxonomic features : the shape and 

 length of the posterior lobe of article 2; the proportional lengths of 



