DERMAL BONES OF FISHES. " 141 



same appendage. In some of tlie earliest introduced fishes on our 

 planet, e.g. the Cephalaspids of the Old Red Sandstone, the oper- 

 cular appendages were functionally as well as homologically cephalic 

 fins, and the only pair of radiated appendages so developed from the 

 htemal arches. 



Returning to the consideration of the dermo- skeleton, we find in 

 the Sturgeon that, besides the cephalic plates, it is represented by 

 five longitudinal rows of dermal bones, one extending along the mid- 

 line of the back {Ji.y. 43, ds) already noticed in the elucidation of 

 the skeleton of the trunk, one along each side of the body (^ib. dp), and 

 two along the lower part of the abdomen, between the pectoral and 

 ventral fins. The upper lateral series of scale bones is pretty 

 constant in the exo-skeleton of fishes, and is usually closely related 

 to the mucous tube and its conduits, which form the so-called ' lateral 

 line' in this class. The systematic Ichthyologist finds in the va- 

 rieties of this line characters for the distinction of genera or species. 

 The lateral bones, which are either perforated or grooved by its 

 ducts, are modified scales, and the scales of fishes are more or less 

 modified dermal bones : they do not belong to the horny or epidermal 

 system, but lie between the cuticle and cutis, their fore margin di- 

 rected inwards and lodged somewhat loosely in depressions of the 

 cutis, and their hind margin outwards, and firmly adherent to the 

 cuticle, when the development of the scales renders its existence 

 possible. The scales of the lateral line are commonly more ossified 

 than those of the rest of the trunk : in the Eel tribe the lateral 

 mucous ossicles are tubular and concealed by the epiderm. In the 

 Sole and Plaice the mucous scale bones of the lateral line are quite 

 superficial. There are many circular radiated ossicles scattered over 

 the dark or upper side of the skin of tlie Turbot. A row of small 

 chevron-shaped dermal bones extends along the median line of tlie 

 belly of the Herring, and the extremity of each lateral process {Jiy. 

 23. dh) is connected with that of the long and slender vertebral rib, 

 completing the inferior arch, like a sternum and sternal ribs. The 

 Dory has two rows of thick osseous plates along the under part of the 

 abdomen ; and both this fish and the Herring have been cited as 

 exceptional examples of fishes with a true sternum. * But the super- 

 ficial position of the ventral ossicles indicates their essentially dermal 

 character, and we may regard this as another instance of the con- 

 nection of the endo- and exo-skeletons in the class of fishes. Parts 

 analogous to a sternum arc thus supi)lied from the exo-skeleton in 

 the Herring, as they are from the splanchno-skeleton in the Lamprey 



* Gore's translation of Cams' Comp. Anat. vol. i. p. 117. 



