NO. 2133. FfSITES TAKEN BY "ALBATROSS," 18SS— THOMPSON. 459 



dium. They are in fact not allied to the latter genus in any respect 

 save in the presence of the dorsal row of scutes and are of very different 

 appearance, probably because of entirely different habits, to which 

 their structure is related. It may be that the dorsal row of scutes is 

 an independent development in each case, in any event it appears 

 to be so in the American species as distinguished from the Austrahan. 

 The following table will serve to distinguish these genera from each 

 other, although in thus comparing them it is not intended to attach 

 any special significance to the fact that they are "double armored" 

 as distinguished from those forms with but a ventral rov/ of scutes : 



a^. Gill rakers very numerous (80 to 150), and long (3 or 4 in head) on lower limb of 

 first arch; a narrow notch between premaxillaries; maxillary broad, its width 

 I5 to If in eye diameter; articulation of mandible behind eye; no teeth; 

 branchiostegals 7 to 10; scales pectinate; head large, compressed, opercular 

 bones deep; eye small and high; ventral inserted under dorsal. 

 b^. A line of scutes between occiput and dorsal; dorsal insertion midway between 

 snout and caudal base; scales in lateral series 50; no angle in lower limb of 



fu'st gill arch Ethmidium (notacanthoides) . 



b^. No line of dorsal scutes, a double row of pectinate scales in their place; dorsal 

 inserted nearer caudal; scales 60 to 80; lower limb of gill arch not angu- 



lated Brevoortia (tyrannus). 



V?. Gill rakers short (7 or 8 in head), and few in nimiber (about 10+25); no notch in 



premaxillary; eye large, not placed high in the head; articulation of mandible 



under center of eye; head small, not much compressed or deej); ventral 



inserted before dorsal. 



&. Maxillary narrow, its iv^idth Ah in eye; branchiostegals 8; teeth in jaws, 



tongue, etc.; scales not pectinate Potamalosa (novae-hollandiae). 



<?. Maxillary broad, its width 2^ to 2\ in eye; l>ranchiostegals 4; no teeth; scales 

 pectinate Hyperlophus (spratellides). 



There have also been three genera of fossil "double-armored" 

 clupeids described. Cope ^ described the genus Diplomystus, dis- 

 tinguished by a series of scutes between the occiput and dorsal, from 

 the Green River Eocene of Wyoming, including forms with very 

 short anal and few vertebrae with the typical species with long anal 

 and many vertebrae. Ogilby ' described the recent genera Ilyper- 

 lo'pTius and Potamalosa ^ for fresh water herrings of AustraUa. Jordan* 

 described two more fossil genera of these forms from Brazil, EJlimma ^ 

 and KnigJitia. Diplomystus is sharply distinguished by its long anal, 

 Ellimma by its few vertebrae and prominent ventral region, and 

 Knightia apparently by the few vertebrae (36). The last-named 

 form may be related to Potamalosa, but tlie identification of such a 

 fossil genus with a recent one is not advisable wlien differences are 

 to be found. 



» Bull. U.S. Geol. Surv. Terr., vol. 3, 1877, p. 800. 

 2 Rec. Austr. Mus., vol. 2, 1892, p. 26. 

 ' Proc. Linnean Soc. N. S. W., vol. 21, 1897, p. 504. 

 ' Annals of the Carnegie Maseum, vol. 7, No. 1, 1910, p. 23. 



s Originally EUipen, preoccupied by FAlipes of Scndder ( Psyclie, 1002, p. .'U)8), later renamed FJUmma by 

 Doctor Jordan. 



