SKELETON OF TELEOSTEI. 87 



visceral skeleton of the skull. Further, a distinction is made 

 between the bones preformed in cartilage and those origiaat- 

 ing ia tegumentary or membranous tissue. It is admitted 

 that the primordial cranium is a coalition of several segments, 

 the number of which is determined by that of the visceral 

 arches, these representing the haemal arches of the vertebral 

 column ; but the membrane-bones are excluded from a consi- 

 deration of the vertebral division of the primordial skull, as 

 elements origiually independent of it, although these additions 

 have entered into special relations to the cartilage-bones. 



With these views the bones of the Teleosteous skull are 

 classified thus : — 



1. Cartilage-hones of the primordial skull. — The lasi-occi- 

 pital (5 in Figs. 23-26) has retained the form of a vertebral 

 centrum; it is generally concave behind, the concavity contaiu- 

 ing remains of the notochord ; rarely a rounded articulary head 

 of the first vertebra fits into it, as in Symh-anchus, and still 

 more rarely it is provided with such an articulary head {Fistu- 

 laria); j5-equently it shows two excavations on its inner surface 

 for the reception of the saccus vestihuli. The exoccipitals (10) 

 are situated on the side of the basi-occipital, and contribute the 

 greater portion of the periphery of the foramen magnum; 

 frequently they articulate with the first vertebra, or meet in 

 the upper median line, so as to exclude the supraoccipital 

 from the foramen magnum. The supraoccipital (8) is interca- 

 lated between the exoccipitals, and forms a most prominent 

 part by the median crest, which sometimes extends far for- 

 wards on the upper side of the skull, and offers attachment to 

 the dorsal portion of the large lateral muscle of the trunk. 

 When the interior portions of this bone remain cartilaginous, 

 some part of the semicircular canals may be lodged in it. 



The region of the skull which succeeds the bones described 

 encloses at least the greater portion of the labyrinth, and its 

 component parts have been named with reference to it by 



