160 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



larger than the right (cp. description of the hyoniandibu- 

 lar), in contradistinction to the hyoniandibular cup which 

 is smaller on this side. The pterotic forms the anterior 

 boundary of the glosso-pharyngeal foramen as shown in 

 fig. 2 (f.gl.). It is bounded by the parietal, sphenotic, 

 prootic, opisthotic and exoccipital. 



Opisthotic or Intercalar (Op.O., figs. 2, 3, 4). — Forms 

 the remainder of the parotic process. It is b}~ far the 

 smallest of the otic bones, consists of a thin flat plate of 

 irregular shape, and contains no cartilage whatever. Its 

 development should therefore be studied. It forms the 

 posterior boundary of the glosso-pharyngeal foramen (f.gL, 

 figs. 2, 3), and is bounded by the basioccipital, pterotic 

 and exoccipital. 



There can be no question in forms such as ihe Cod 

 and Plaice that the ear bones described as pterotic and 

 sphenotic are something more than what they seem, i.e., 

 they have a compound and not a simple origin. Added to 

 the so-called cartilage bone in each case is a dermal 

 element, originally developed around that part of the 

 sensory canal system associated with these bones, and now 

 more or less completely fused on to them. In some Fishes 

 (such as the sphenotic of Amid) the two portions remain 

 distinct throughout life, and in others the line of fusion 

 may be plainly seen, with, however, the bones remaining 

 separate as an occasional abnormality (such as the pterotic 

 of the Cod). But as a rule the two portions fuse com- 

 pletely, so as to be indistinguishable in the adult. Now 

 in the one case the terms pterotic and squamosal have 

 been applied indifferently to the compound of the adult. 

 We may therefore, in those cases where the two parts of 

 the compound remain separate in the adult, call the car- 

 tilage pterotic, or ear bone, the true pterotic, and the 

 dermal pterotic, or lateral line bone, the squamosal. In 



