82 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



longation of the processes on the anterior sides of the pharyngobranchials; these newly 

 prolonged processes then became segmented off and articulated with the epibranchials in 

 the same way that the entire branchial arch originally became segmented. Thus the 

 ^-shaped curve would be changed into a > -shaped one. 



Origin of the Orobranchial Apparatus of Gnathostomes 



In the cephalaspid ostracoderms (cf. Fig. 2B) as described by Stensio (1927) the first 

 six gills of the ten gill sacs extend directly laterad, having small round external openings 

 as in the lampreys. In the latter the branchial basket is a delicate stiffening skeleton of 

 cartilage, without joints. In the primitive shark (Figs. 5, 6) as described by Allis (1923a) 

 the jaws and gill slits are directed obliquely backward, the stout branchial skeleton is 

 elaborately segmented and jointed; the jaws and branchial arches are surrounded externally 

 by band-like constrictor muscles (Cs I, 2, 3, etc.) and there are adductor and interbranchial 

 muscles on the arches themselves (Fig. 4). 



Fio. 5. Underside of the skull of a Devonian shark, CWojWaf A* /y/^ri. After Dean. 



Based on several well-preserved specimens and showing the mandible in series with the lower bars of the branchial arches; 

 the median pieces are not shown. 



