﻿REVIEW OF GENUS DOROSOMA — MILLER 391 



The characters used to distinguish Dorosoma and Signalosa are 

 summarized in table 1. 



The only other New World clupeid likely to be confused with either 

 Dorosoma or Signalosa is the wholly marine genus Opisthonema. 

 Signalosa appears to be more marine in its habitat preference than 

 Dorosoma, but even so it would probably only rarely occur with 

 Opisthonema. In all three genera the last ray of the dorsal fin is pro- 

 longed into a prominent filament. Beyond this the resemblance is not 

 great, but a further comparison seems desirable. Some of the follow- 

 ing characters were worked out in collaboration with the late Dr. S. F. 

 Hildebrand while he was studying the Western Atlantic Clupeidae, 

 exclusive of the gizzard shads. Opisthonema differs from both Doro- 

 soma and Signalosa in having (1) the ridge of the back anterior to 

 dorsal origin crossed by scales rather than naked; (2) a bilobed, dermal 

 fold on the vertical anterior edge of the cleithrum; (3) a weak develop- 

 ment of the paired pharyngeal pockets (Lagler and Kraatz, 1945) 

 found above the branchial cavity of the gizzard shads; (4) a broader 

 upper jaw, so that the maximum width near the distal portion is 

 greater than (rather than less than) the diameter of the pupil; (5) no 

 axillary scale, the pectoral fins folding into a groove; (6) the opercle 

 and subopercle elongated and narrowed so that the maximum ^vidth 

 of either bone enters the length of the opercle 1.6-2.4 (rather than 1.1 

 to 1.5, rarely 1.6) times; (7) the dorsal origin well in advance of 

 the pelvic insertion (over or well behind in Dorosoma, very slightly in 

 advance to behind in Signalosa) ; and (8) the gizzard is smaller and 

 more elliptical. Atlantic material of Opisthonema was examined from 

 North Carolina to Brazil, including Bermuda, the West Indies, and 

 Panama; Pacific specimens studied came from Mazatlan to Peru, 

 including the Galdpagos Islands. 



The following key will serve as a convenient means of rapid determi- 

 nation of the species of Dorosoma. Greater detail is given under the 

 separate treatment of each species and in the tables. 



KEY TO THE SPECIES OF DOROSOMA 

 la. Lateral scales 52 to 70, usually 58 to 65; scales around body 36 to 45; vertebrae 

 48 to 51. Atlantic drainage of eastern North America south to Rio 



Pdnuco Basin, Mexico cepedianum 



\h. Lateral scales 70 to 83, usually 73 to 78; scales around body 46 to 60; vertebrae 



43 to 48 2 



2a. Dorsal filament long, its length as measured from dorsal origin 0.95 to 1.4, 

 usually 1.1 to 1.3, in distance from pelvic insertion to tip of snout; anal 

 base 1.1 to 1.4 in same distance; anal rays 29 to 38, usually 32 to 35. 

 Atlantic slope of Mexico and northern Guatemala (Rfo Papaloapdn to 



Rio Usumacinta) anale 



26. Dorsal filament short, its length as measured from dorsal origin 1.4 to 1.85, 

 usually 1.5 to 1.8, in distance from pelvic insertion to tip of snout; anal 

 base 1.6 to 2.2 in same distance; anal rays 22 to 31, usually 23 to 29__3 



