ANATOMY OF HETERODONTUS: ENDOSKELETON 463 
hypobranchials and the fifth ceratobranchials are attached. 
This piece bears a posterior segment. 
The discovery of a vestigial sixth branchial arch in the Hetero- 
dontidae (Hawkes ’05) has been of considerable interest. This 
arch was found in the young both of Heterodontus francisci 
(Gyropleurodus francisci) and of H. philippi. Since this dis- 
covery, however, an additional rudimentary arch has been 
described in several other elasmobranchs. 
Text-fig. B Rudimentary sixth branchial arch of Heterodontus francisci; 
Cr.Br.5., fifth ceratobranchial cartilage; Cr.Br.6., sixth or rudimentary cerato 
branchial cartilage; Hp.Br.4., fifth epibranchial cartilage; Ep.Br.6., sixth or rudi- 
mentary epibranchial cartilage; Lg., ligament passing under pit; pt., pit in neck 
of pharyngobranchial cartilage. 
The rudiments of the arch found by Hawkes in Heterodontus 
francisci consist of a pair of pieces located back of the fifth arch. 
The upper piece is attached by a ligament to the epibranchial 
segment of the fifth arch and is joined below to the second rudi- 
mentary segment. These pieces are interpreted by Hawkes 
either as the cerato- and hypobranchial, or as the epi- and 
ceratobranchial segments which have become closely joined to 
the fifth arch. 
In the adult specimen these cartilages (ep.br.6 and cr.br.6, 
text-fig. B, and fig. 14) are much like those described for the 
