1904.] OSTEOLOGY OF THE ELOPID.E AND ALBULID^. 59 



particular character. The persistence of the interfi'ontal suture 

 in the different genera of the Mursenidse, for instance, is veiy 

 inconstant. 



In Osteoglossum the mesethmoid is separated from the frontals 

 by the meeting of the two nasal bones in the middle line. In 

 Megalops the endosteal mesethmoid is more firmly united with the 

 vomer than with the ectosteal mesethmoid ; in Arapaima the 

 endosteal mesethmoid alone is present, and this does not present 

 itself on the loof of the ci'anium ; while in Chanos and the 

 Salmonidfe an ectosteal mesethmoid is present, and the cartilage 

 is unossified. 



It is the rule among the Malacopterygian fishes for the two 

 exoccipital bones to meet above the foramen magnum, and for the 

 basioccipital to be exclvided fi-om the floor of the brain-cavity by 

 the union of the two exoccipitals below the brain. The two pro- 

 otics also unite beneath the brain, and foi-m, with or without 

 the co-operation of the basisphenoid, the roof of the eye-muscle 

 canal. 



The supraoccipital crest is evidently to be regarded as an osseous 

 sheet developed in relation with the great trunk-muscles, and not 

 a backward extension of the supraoccipital bone itself. Chanos 

 is very interesting in this respect, in that it shows a condition 

 intermediate between the usual vertically disposed sheet of bone 

 and the separable brush-like tendon-bone or intermuscular bone 

 that projects back from the supi'aoccipital proper in Chatoessus. 

 In the latter genus similar and separable osseous brushes project 

 back from the epiotic bones ; and in SphyroeAia and Mugil, Starks 

 (Proc. U.S. Nat. Mus. xxii. 1900) has described and figured 

 similar brushes, but not separable, continuous with the back of 

 the epiotic bones. Osseous brushes on the back of the exoccipital 

 bones are of much more common occurrence, and these are dis- 

 cussed on p. 65. 



The extent to which the supraoccipital, epiotic, and squamosal 

 crests project backwards, and to which the hinder surface of the 

 cranium is excavated for the better attachment of the trunk- 

 muscles, may be taken more or less as a measure of the 

 specialisation of the skull, since in the pre-Cretaceous bony fishes 

 the back of the cranium is nearly flat. The excavation of the 

 back of the cranium has doubtless originated independently in 

 different groups ; and AUis has pointed out (Zool. Bull. ii. 2, 1898, 

 p. 92) tha,t this must certainly have been the case in Amice and 

 Scomber, for in the former the trunk-muscles have extended 

 forward beneath the parietal bones, wheieas in the latter they 

 lie externally to the pai'ietals. The degree of irregularity of the 

 back of the skull and the dimensions of the posterior temporal 

 fossae appear to be in direct proportion to the muscularity of the 

 front portion of the trunk of the fish. 



The orbitosphenoid is a bone which is very variable in its form 

 and occurrence ; it is wanting in Osteoglossu-rn, Gonorhynchus, and 

 Chanos. In the majority of the lowei- Malacopterygian fishes it 



