GREGORY: FISH SKULLS 



155 



referred to the second grand division (Otophysi). This main division rests on the entirely 

 unproved assumption that the caecal diverticula of the air bladder, which in the Clupeidae 

 extend forward into the pterotic and prootic bones, are fully homologous with somewhat 

 similar diverticula found in the Ostariophysi. But this assumption has already been 

 disputed for apparently good reasons by Ridewood (1904c, p. 214). Moreover, numerous 

 figures of the skulls of Clupeidae, Chanos, Engraulis, Salmo, etc., assembled in the present 

 work, support the more conservative conclusion that in spite of their retention of an adipose 

 dorsal fin the Salmonidse are a modern offshoot of the old clupeid-elopid stock, which runs 

 back through the leptolepids into early Mesozoic times. 



sphot P^ T 



alsp i^^^ ^PM 



_..-hyom 



deth*na^la 

 k eth 



va*eth 

 la 



Cromeria 



Fig. 49. Cromeria nilotica. Ahtr Swinnerton. 



Cromeria. — This is a scaleless, diminutive fish from the Nile, of uncertain affinities; its 

 osteology has been carefully described by Swinnerton (1903). That it is an isospondyl of 

 some sort is certain, as shown by many features, including the retention of a mesocoracoid 

 arch and of an air bladder and pneumatic duct similar to that of Salmo; it also lacks 

 Weberian ossicles (Swinnerton, 1903, p. 59). The general characters of its cranium, jaws 

 and opercular system also conform to the isospondyl type. 



Many of the skull characters are highly specialized. The suspensorium extends well 

 forward and the mouth is small and edentulous, with very small premaxillas and laterally 

 placed maxillae forming the greater part of the oral border. The hyomandibular has an 

 extraordinarily large anterior process out of all proportion with the reduced metapterygoid. 

 The symplectic is absent. The large lacrymal is fused with the mesethmoid and nasals. 

 Seen from above, the skull is elongate, with greatly swollen, rounded braincase, very sug- 



