DERMAL BONES OF FISHES. 



131 



cartilaginous basis of the skull oppose the view of its homology with 

 the basal elements of the cranial vertebra;. With regard to the 

 upper and lateral osseous jilates of the head, they are, as Von Baer 

 has indicated (lu.), the continuation of the series of dermal osseous 

 plates upon the upper mid-line and sides of the trunk. 



Before, however, applying this instructive condition of the cranium 

 of the Sturgeon to the elucidation of the nature and homologies of 

 the bones which still remain to be noticed in the skull of the Cod, I 

 shall briefly describe the maxillary, mandibular, hyoid, and scapular 

 haemal arches of the coalesced cranial vertebrae of the Sturgeon. 

 The first three of these arches are suspended from the tympanic 

 pedicle ; but this, instead of being a single piece, as in the Plagiostomes 

 and Lepidosiren, consists of three cartilages, articulated in Accipenser 

 Ruthenns, according to Miiller, by a small accessory or interarticular 

 cartilage, with the under part of the mastoid process. The three 

 principal bones describe a semicircle, concave forwards, and answer 

 respectively to the epi-tympanic {fig. 43. 25), the meso-tympanic {ib, 

 26), and hypo-tympanic {ih. 28) bones of Osseous Fishes. 





Foro-part of encio- and exo-skcleton of Sturgeon. 



The upper jaw, or maxillary arch of Plagiostomes and Lepidosiren, 

 is represented in the Sturgeon by a partly osseous, partly cartila- 

 ginous, broad arch, in which the centres of ossification have been 

 three in number on each side, and indicate both by their relative 

 position, and by the direction in, and extent to, which the bony 

 fibres have diverged from them, tlie pre-maxillary, maxillary, and 

 palatal bones of the Osseous Fishes. The pre-maxillaries {ib. 22) 

 form the anterior and inferior border of the arch : each bone is a 

 sub-triangular plate, joined by ligament to its fellow at the middle 

 line, trenchant anteriorly, contracted and thickened posteriorly, 

 whence it rises and extends in the form of an arched process, out- 

 wards and downwards to the outer side of the joint for the lower 



K 2 



