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PROCEEDINGS OF THE NATIONAL MUSEUM 



As previously stated under the account of chavesi, it is thought that 

 these similarities may indicate a common ancestry. On the other 

 hand, it is perhaps equally plausible to consider the resemblances 

 between smithi and chavesi to be the result of parallel evolution. Under 

 this alternate hypothesis it is suggested that the gizzard shad may have 

 gained access to the Pacific drainage during Tertiary times, when a 

 continuous waterway connected the Atlantic and Pacific across 

 what is now the Isthmus of Tehuantepec. This idea is supported by 

 the absence of any records of Dorosoma on the Pacific slope of Middle 

 America south of the Isthmus of Tehuantepec. To the north of that 

 region Dorosoma is known at present only as far south as Sinaloa, 

 Mexico, though I venture to predict that thorough exploration along 

 the Coastal Plain will reveal its occurrence much farther southward. 

 Careful ichthyological surveys in El Salvador (Hildebrand, 1925) and 



Table 8. — Proportional measurements of 27 specimens of Dorosoma smithi 

 expressed in thousandths of the standard length 



[Superscripts indicate the number of specimens; averages are given in parentheses. Localities are arranged 

 from northwest to southeast (see table 9)] 



Measurement 



Standard length, in mm 



Dorsal origin to tip of snout 



Body: 



Greatest depth 



Greatest width 



Head: 



Length 



Depth 



Width 



Interorbital, least fleshy width 



Eye, length 



Snout+eye 



Dorsal filament, length... 



Dorsal fln, basal length 



Pectoral fin, length 



Lower caudal lobe, length 



Species 

 total 



" 82-134 

 (107) 

 475-528 

 (502) 



319-399 

 (301) 

 98-135 

 (115) 



278-321 



(297) 



215-255 



(235) 



106-138 



(126) 



66- 93 



(75) 



62- 77 



(70) 

 115-136 

 (125) 

 131-239 

 (186) 

 121-141 

 (131) 

 191-230 

 (209) 

 288-360 

 (323) 



*Predorsal region of 1 specimen abnormal; value not used (see Hubbs and Miller, 1941, table 1). 

 for other 2 was 484. 



Value 



