DERMAL DONES OF FISHES. 13o 



supra-cranial movable plates (^fig. 27. 12) are the only bones of the 

 head of the Lepidosiren which can be referred, with any probability, 

 to the dermal system. It is plain that the subjacent epicranial plate 

 (^fig. 27. 11) in close connection with the cartilage of the cranium, is 

 a true part of the endo-skeleton, and is as certainly the homologue 

 of the mid-frontal, parietal, and supra-occipital bones. In the de- 

 velopment of the skull of Osseous Fishes it is found, however, that, 

 whilst the central or basilar, the neurapophysial, and the parapophy- 

 sial elements of the cranial vertebrae are developed out of a pre- 

 existing cartilaginous basis, the modified spinous elements, with the 

 exception of that of the occipital vertebra, are formed by the depo- 

 sition of the calcareous salts in the epicranial membrane ; and Dr. 

 Reichert, apparently not remembering that the cartilaginous, or in- 

 termediate histological, change between the primitive membranous 

 and ultimate osseous stage has been as little recognised in the de- 

 velopment of the epicranial bones of Man, would reject the parietal 

 and frontal bones from the system of the endo-skeleton. 



To those who may be inclined to support this view, by reference 

 to the epicranial dermal plates in the species of Sturgeon Avhere their 

 correspondence with the mid-frontal and parietal bones may be most 

 easily recognised, it may be replied, that the pre-frontals, post- 

 frontals, mastoids, and supra-occipitals, might also be referred to the 

 exo-skeleton, by a like reference to dermal plates holding the con-e- 

 sponding positions in the Sturgeon's head : but the skeleton of the 

 Lepidosiren, with the known relations of the pre-frontals, post-fron- 

 tals, mastoids, and supra-occipitals to the primitive cartilaginous basis 

 of the skull in Osseous Fishes, demonstrate the fallacy of the conclu- 

 sions as to the dermal origin of the frontals and parietals, based upon 

 the deceptive analogies of the dermo-cranial plates in the Sturgeon, 

 and upon the absence or brief duration of the cartilaginous stage in 

 the ossification of certain expanded spines of the cranial vertebras. 

 That the homologues of some of the dermal plates in the Sturgeon 

 are retained in the skull of Osseous Fishes is, however, rendei'ed 

 extremely probable by the constancy of their relative position, by 

 their development in a dermal basis, and by their relation to the 

 dermal mucous canals. 



The accessary bones of the skull in Osseous Fishes, which I regard, 

 on the above grounds, as appertaining to the exo-skeleton, and which 

 are more especially connected with the mucous organs of the skin, 

 are the sub-orhital, the siipra-orhitul, and the supra-teniporfil 

 ossicles. Tlie first sub-orbilal bone {Jig- 19. 73) is always the 

 lai'gest : it is triangular in the Cod, and covers the side of the muzzle, 

 extending from the fore part o\' Ihc orbit to Ihe anterior cud of 



K 4 



