THE CHONDKOCRANIUM IN THE ICHTHYOPSIDA. 123 



dorsal view of the skull and drawings of sections through 

 various parts of the head. 



No cartilaginous occipital arch is present, owing, ap- 

 parently, to ossification. The otic capsule is large and 

 selachian-like in form. In its postero-lateral wall there 

 is a large aperture exposing portions of the posterior and 

 horizontal canals. This also is prol)ably due to ossifica- 

 tion. A small remnant of the hyomandibular cartilage (h) 

 lies in a groove in the dorso-lateral surface of the capsule. 

 That it formerly reached down as far as the posterior end 

 of the pterygo-quadrate cartilage is shown by the figures 

 of Pollard and others. 



A thick synotic tectum covers the brain cavity in the 

 posterior two-thirds of the otic region. There are indica- 

 tions of a medial capsular wall separating the brain cavity 

 from the cavity of the capsule at its posterior and anterior 

 ends, but the gieater part of the space is entirely open. 

 The floors of the capsules are continuous with the basilar 

 plate which slightly exceeds the synotic tectum in extent. 

 A peculiar rod of cartilage (bo) projects a short distance 

 backward from beneath the middle of the basilar plate. 

 At the anterior end of the capsules there are upon each 

 side two foramina and a deep groove which is now open 

 in front and apparently represents another foramen the 

 anterior wall of which is ossified. A short bar of cartilage 

 passes across the external opening of the posterior of the 

 two foramina, dividing it here into two. The unossified 

 posterior end of the supraorbital band remains as a solid 

 lateral projection upon the anterior end of the otic cap- 

 sule. 



There is a complete separation of the chondrocranial ele- 

 ments of the otic region from those of the orl)ital and 

 nasal regions. The supraorbital band (sb) passes an- 

 teriorly into a broad tegmen cranii (fc) which covers over 



