THE SKULI. OF FISHES. 83 



Behind the tympanic pedicle is the pre-opercuUir bone {juj. 27. 

 34), elongated, pointed at both ends, triedral, with the outer sur- 

 face concave : its lower two-thirds is attached by ligament to the 

 mandibular or tympanic pedicle. Behind and below this is an in- 

 equilateral triangular bone {ih. 37) closely attached by ligament to 

 the expanded cranial end of the hyoidean arch : this I originally 

 described as the styloid bone ; it may be the homologue of the inter- 

 opercular.* 



Only a single ' cerato-hyoid' {ih. 40) is ossified on each side : they 

 complete the arch by the ligamentous junction of their lower extre- 

 mities, having no intervening basi-hyal : their upper expanded ends 

 are suspended by a short ligamentous mass to the cartilage imme- 

 diately behind the tympanic pedicle. 



The capsules of the organs of sense are of nearly equal size ; the 

 eye is the smallest ; the nose the largest. The acoustic capsules are 

 principally buried in the lateral cartilages of the skull ; but one of 

 the otolithes protrudes through a moderately wide hole into the cranial 

 cavity. The eye-ball occupies the space between the pre- and post- 

 frontals above, and the outward prolongation of the maxillary below; 

 its capsule, the sclerotic, is cartilaginous. The nasal capsules (ih. 19) 

 are also cartilaginous, with vertical slits closed by membrane ; they 

 are situated on each side and below the nasal plate. 



You may perhaps think that I have been biased by the extrinsic 

 interest of personal discovery, in dwelling so long upon the cranial 

 structure of the Lepidosiren ; but I persuade myself that the actual 

 value and intrinsic importance of this remarkable type of Ichthyic 

 organisation, will justify the time and attention we have been be- 

 stowing upon it. The skeleton of the Lepidosiren affords the right 

 key to the complexities of those of the typical and better ossified 

 fishes. I believe it to manifest, upon the whole, the highest grade 

 which is attained in the class of Fishes, in the direct progress to 

 perfection, or, in what may be termed the Vertebrate high road. 



The true or typical osseous fishes deviate from this I'oad into bye- 

 paths of their own, and superadd endless complexities of which we 

 shall seek in vain for liomologous parts in Reptiles, Birds, or 

 Mammals. Therefore it is, that, on the whole, the Lepidosiren's 

 skeleton presents the closest resemblance to that of the lowest class 

 of Reptiles, though it differs therefrom both by a little less and a 

 little more development : the vertebral centres of the trunk, for ex- 

 ample, have not risen above the embryonic state of soft confluence ; 

 but secondary spines have been superadded to the neural and ha'mal 



* Agassiz regards the pre-opereular in fishes as the liomologue of the styloiJ. 



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