GREGORY: FISH SKULLS 137 



For the purposes of this study the Isospondyli may be divided into six superfamilies by 

 modifying slightly the arrangement adopted by Tate Regan (1929, p. 313). 



Superfamily Clupeoidea: Leptolepidae, Elopidae, Albulidae, Chirocentridae (Sauro- 

 dontidae), Clupeidae, Ctenothrissidse, Alepocephalidae, Chanidse, Kneriidae, Phractolaemidse, 

 Cromeriidae. 



Superfamily Salmonoidea: Salmonidae, Microstomidae, Argentinidae, Opisthoproctidae, 

 Osmerid^, Salangidas, Retropinnatidse, Haplochitonidae, Galaxiidae. 



Superfamily Stomiatoidea: Gonostomatidae (including Maurolicus, Gonostoma, etc.), 

 Sternoptychidae, Astronesthidae, Chauliodontidae, Stomiatidae. 



Superfamily Osteoglossoidea: Osteoglossidae, Pantodontidae. 



Superfamily Mormyroidea: Hyodontidse, Notopteridae, Mormyridae, Gymnarchidae. 



Superfamily Gonorhynchoidea: Gonorhynchidae. 



It is not necessary to define these tentative superfamilies. They are recognized simply 

 for convenience in expressing the apparent interrelationships of this enormously variable 

 series of families. 



The researches of G. Allan Frost (1925-1930) on the otoliths of the Neopterygian 

 Fishes add an important and practically new set of criteria for estimating the relationships 

 of the suborders and families of teleosts. Many of Mr. Frost's conclusions are noticed 

 below. 



We may now make a brief survey of the skull structure of these groups, supplementing 

 our own very limited material by constant reference to the works chiefly of Smith Wood- 

 ward and Ridewood for the cranial osteology of the leading fossil and recent types. 



Clupeoidea (Albulids, Tarpon, Herrings, etc.) 



Leptolepids. — In the Jurassic and Cretaceous Leptolepis (A. S. Woodward, 1895, p. 

 501) the isospondyl skull is seen in its most typical and primitive form, without any of the 

 aberrant specializations of later types (Fig. 30). Ganoidean reminiscences, however, are 

 not wanting in the more or less enamelled condition of the delicate membrane bones of the 

 head; the centra, though well ossified, are pierced by the notochord, the scales, though 

 thin, cycloidal and deeply imbricating, usually remain ganoid in structure on their exposed 

 portion. The maxillae, bearing two prominent supramaxillse, as in the clupeoid group, are 

 now wholly freed at the posterior end and loosely articulated anteriorly. 



The large head and large eye, the somewhat upwardly directed and fairly large mouth 

 bordered with minute teeth, suggest that these fishes were herring-like also in habits, 

 feeding in schools on the plankton of the Jurassic and Cretaceous seas. The broad, 

 rounded preopercular and large circumorbital plates suggest the Elopidae. According to 

 Frost (1925a, p. 153; 1926, pp. 82, 83), the otoliths of the-Elopida "are so similar to those 

 of their Jurassic prototypes, the Leptolepidae, that they confirm in a striking manner Mr. 

 Tate Regan's opinion that these should be placed together as one family." The otolith 

 named by Frost "Otolilhus {Leptolepidarum) rostratus n. sp." from the Upper Jurassic of 

 England resembles the otoliths of the Elopidae in general appearance, but differs in certain 

 details. 



Elopids. — The Elopidae, including the "Ten-pounder" (Elops) and the tarpon {Mega- 

 lops, Tarpon), are regarded by both Woodward and Ridewood as "the most archaic of 

 existing Teleosteans." Elops and Megalops both date from the London Clay (Lower 



