NO. l8 MENHADEN, GENUS BREVOORTIA HILDEBRAND 5 



longer in large specimens than in half-grown ones. This development 

 makes it necessary to compare specimens of the dififerent species of 

 nearly equal size as already indicated, to show actual differences in 

 this respect. It is important, also, to compare scales from the same 

 part of the body, as the serrations are not uniformly developed on all 

 parts. In general, the serrae are larger on the scales from the back 

 than those from the lower part of the side. The illustrations (fig. 8) 

 given are based on scales of adults taken from the middle of the side 

 below the anterior rays of the dorsal. 



Relationship of the species. — The seven species of American men- 

 haden roughly fall into two groups on the basis of scales. One group — 

 tyrannus, hrevicaudata, patronus, pectinata, and aurea — has moder- 

 ately large scales, which are arranged in fairly regular series on the 

 sides of the body, and the other group — smithi and gunteri — has 

 smaller and more irregularly placed scales. The species may also be 

 divided into two groups on the shape of the ventral fins, tyrannus, 

 hrevicaudata, and patronus having rounded fins in which the inner- 

 most ray is not much shorter than the outermost one, while smithi, 

 gunteri, pectinata, and aurea have fins with nearly straight (some- 

 what convex in aurea) oblique margins in which the innermost ray 

 is much shorter than the outermost one (see fig. 9), giving the fin a 

 pointed appearance when folded. Also the South American species 

 may be separated from the North American ones by the smaller re- 

 duction in the size of the scales on the back and on the base of the 

 caudal in comparison with the scales on the middle of the side. 



The North American species fall into two closely related pairs, 

 namely, tyrannus from the Atlantic and patronus from the Gulf, and 

 smithi from the Atlantic and gunteri from the Gulf, and one odd one, 

 hevicaudata, from Noank, Conn., related to tyrannus. The close rela- 

 tionship between the species of each pair named is not confined to 

 Brevoortia, as a similar relationship exists between the shad, Alosa 

 sapidissima, of the Atlantic, and A. alabamae of the Gulf, and also 

 between Pomolobus mediocris of the Atlantic and P. chrysochloris of 

 the Gulf and Mississippi Valley. Nor is such a relationship limited 

 to the family Clupeidae, as two similarly closely related pairs are 

 known to exist in the family Sciaenidae, namely, Cynoscian regalis 

 of the Atlantic and C. arenarius of the Gulf (Ginsburg, 1929, p. 83), 

 and the other pair occurs in the genus Menticirrhiis (also discovered 

 by Ginsburg, unpublished). 



In each pair of the fishes named, the range very probably once was 

 continuous, but became discontinuous when the last passageway for 

 fishes across the Florida peninsula became closed. None of the fish 



