38 
also obvious that the basal segment of this arch (C.Br.*) 
is serially homologous with the cerato-branchials, the 
missing element therefore being the hypo-branchial. 
Hence the fourth basi-branchial may represent either the 
two vestigial hypo-segments fused together (a primary 
basi-segment being absent) or it may have been formed 
by the basi- and two hypo- elements fusing up. 
Branchial Arch IY—The cerato-branchial (C.Br. ) 
is slightly grooved ventrally. The epi-branchial (#.Br.*) 
is a stout L-shaped bone, and is strongly connected with 
the same segment of the preceding arch. The pharyngo- 
branchial (P.Br.*) is small, and bore six teeth. 
Branchial Arch ¥.—This is more reduced than any 
of the other arches, and consists of a single bone on each 
side in which there are practically no traces of asymmetry. 
This is the inferior pharyngeal bone of Cuvier, and 
appears to represent the cerato-branchial segment only of 
the arch.* The inferior pharyngeals (J.Ph.) are stout 
triangular-shaped bones separate dorsally but bridged in 
front ventrally by a tract of cartilage. Two irregular 
rows of teeth are borne on the pharyngeal surface, and in 
the specimen now described there were 12 on the ccular 
side and 14 on the eyeless. At the side and at the base of 
the outer row are situated the replacing teeth, which be- 
come functional as their predecessors wear away. 
Gill Rakers.— These diminish both in size and 
number from before backwards. Their function is to pro- 
vide a rough filtering apparatus for the water passing out 
of the pharynx. ‘Their small size and number in the 
Plaice is due to the nature of the fishes’ food. In those 
fishes where the food might easily escape through the gill 
slits (e.g. Clupea), the gill rakers are much longer and more 
numerous. They are purely dermal and are not fused 
*Cp. Cunningham, op. cit., and W. K. Parker, Phil. Trans., 1873 (Salmo). 
