Fishes of the JVestern Nortli Atlantic 9 5 



that are easily separable typically may intergrade with one another marginally. For 

 instance, the presence of parietal bones among the Clupeoidea and not among the 

 Stomiatoidea has been invoked as an alternative between these two Suborders, but 

 this is not strictly so, since some stomiatoids do have parietals (see Regan and Tre- 

 wavas, ^J: figs. 6, 9, 10; Giinther and Deckert, jp: 244). Neither is the arrangement 

 of the photophores on the sides in definite longitudinal rows reliably diagnostic for the 

 Stomiatoidea, since they are not so in one species of Neonesthes, while in one other 

 species the linear arrangement is to be recognized only with difficulty,^* as is true of 

 some species of Astronesthes as well.-' And Mrs. Marion Grey contributes the infor- 

 mation that some species of Cyclothone, Gonostoma, and Malacosieus have no photo- 

 phores at all on the sides of the body. 



Under these circumstances the construction of a satisfactory key to the Suborders 

 of Isospondyli that would withstand the test of time is not to be hoped for at present. 



Tentative Key to Suborders of Isospondyli of the Western 



North Atlantic 



I a. Parasphenoid bone on roof of mouth with low close-set teeth; larval stage ribbon- 

 shaped (leptocephalus-like). Elopoidea, Part 3, p. 107. 

 lb. Parasphenoid bone without teeth; larval stage in most not ribbon-shaped.^* 



2a. Premaxillary bones minute; maxillaries forming almost entire length of upper 

 jaw; branchiostegal rays plate-like, the edge of the uppermost attached to 

 lower margin of subopercular bone and thus taking part in formation of gill 

 cover. Bathylaconoidea, Part 4. 



2b. Premaxillary bones well developed; branchiostegal rays not plate-like; the 

 edge of upper branchiostegal ray not joining subopercular and interopercular 

 bones and thus not taking part in formation of gill cover. 

 3a. Luminescent organs (photophores) absent in most; never present on head, 

 but present on eye-tube in a few {Dolichopteryx of Argentinoidea) ; if 

 present on sides of body they are in nonlinear arrangement except along 

 base of anal fin {Binghamichthys of Alepocephalidae) ; some with a tubular 

 papilla on each shoulder that connects with a voluminous sac beneath 

 the skin (Searsiidae). 

 4a. No adipose fin between rayed dorsal fin and caudal. 



5 a. Snout noticeably flattened dorsoventrally (depressed), in duck- 

 billed form; teeth along rear part of lower jaw large, formidable, 

 conspicuous; premaxillary bones far apart in front; mesethmoid 

 (proethmoid) bone paired. Esocoidea, Part 4. 



26. Information contributed by Robert H. Gibbs. 



27. According to Zugmayer {$6: 4), the photophores are distinguishable only with difficulty in Astronesthes ntger (as 

 A. myriaster), but Regan and Trewavas {42: 20, fig. 12) picture them clearly for the type specimen of that species. 



28. Certain of the Stomiatoidea have ribbon-shaped larvae; Part 4. 



