no VERTEBRATE SKELETON 



necessary to distinguish them. Prootic and epiotic are constant. 

 The prootic (petrosal) the largest of these, forms the floor and most 

 of the lateral wall of the labyrinth, the bones of the two sides usually 

 fusing in the middle line behind the hypophysis and dorsal to the 

 parasphenoid. The epiotic (occipitale externum of older works) 

 forms the postero-lateral angle of the cranium, extends to the base 

 of the skull and bounds the posterior semicircular canal, besides 

 affording attachment for the supracleithrum. The opisthotic, more 

 ventral in its position, is often absent; when present it is frequently 

 excluded from the labyrinthine wall. The sphenotic is often, next 

 to the frontal, the most conspicuous bone of the dorsal surface, and, 

 although a cartilage bone, is often called postfrontal. Its lateral 

 surface in most fishes is grooved for a part of the hyomandibula, and 

 it is overlaid by a dermal bone, possibly the true postfrontal. The 

 pterotic ossifies in the capsular wall above the lateral semicircular 

 canal and is covered externally by the squamosal, the two frequently 

 fusing to a squamoso-pterotic bone, the squamosal part being 

 traversed by a lateral hne canal. When the sphenotic is small, the 

 squamosal may articulate with the frontal, from which it is usually 

 separated by the parietal. 



In connexion with the otic capsule it may be noted that frequently the inner 

 ear contains, instead of the minute otoHths of most Vertebrates, one or more large 

 calcifications ('ear-stones'), irregular in shape and particularly large in 

 Sciaenoids. 



When fullest developed the pterygoquadrate arch (fig. 103'! is a 

 series of bones, some dermal, some cartilage in origin. The series 

 has been outhned above, (p. 71), and the following statements are 

 additional. The ectopterygoid (epipterygoid) is on the ventral 

 side of the pterygoid process and below the ento- (meso-) pterygoid. 

 The ectopterygoid usually extends along the anterior border of the 

 quadrate, but sometimes a part of the entopterygoid intervenes. 

 There is an intimate relation between pterygoquadrate and hyoid 

 arches, both metapterygoid and quadrate being closely associated 

 with the hyomandibula, the first directly, the other usually by the 

 intervention of the symplectic, although a direct connexion may 

 occur. The articulare is the only cartilage bone known in the lower 

 jaw. It is usually coossified with the goniale, the distinction between 

 the two being recognizable only in development, but they may remain 

 separate in many Clupeids. 



