18 w 2 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



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-branehial 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 (E.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 Y. — 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 (I. 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 ocular 

 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 pharjoix. 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). 



