424 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



cerato-branchials, or the lower branchial elements, are present in many groups of fishes. 

 In the carps and their allies the fifth ceratobranchials bear variously shaped processes which 

 oppose a horny pad that rests on a bony projection from the basi-occipital. These pharyn- 

 geal teeth are drawn upward against the pad by powerful muscles that are attached to the 

 lateral fossa of the cranium. In certain cyprinodonts (Microcyprini) the opposite toothed 

 lower pharyngeals are united by a dentate suture and together form a triangular plate that 

 opposed the teeth on the upper pharyngeals. In the skippers and flying-fishes the opposite 

 lower pharyngeals are coalesced into a solid triangular block (whence the name Synentogna- 

 thi). A more or less similar pharyngeal dental apparatus is developed in the pomacentrids, 

 cichlids, chaetodonts, embiotocids, labrids. In the scarids (Fig. 134) this pharyngeal mill 

 becomes very elaborate, the elongate upper many-ridged plate sliding back and forth on 

 the parasphenoid, while the lateral processes of the fused lower pharyngeals fit into grooves 

 on the inner side of the cleithra. 



Among the more remarkable types of teeth developed in the front of the mouth we 

 may recall the incisiform front teeth of certain sparids (Fig. 123) and of Gobiesox (Fig. 249), 

 the immensely long sabre-like lower tusks of Chauliodus (Fig. 55), the bristling front teeth 

 of pomacentrids (Fig. 128), Blennius (Fig. 252) and Chatodon and the beak-like pincers, 

 presumably developed from the fusion of many bristle-like teeth, in the scarids (Fig. 133), 

 hoplegnathids (Fig. 124), siganids (Fig. 159), tetraodonts (Fig. 169), diodonts (Fig. 171). 



The edentulous condition has been developed independently in various groups. In 

 Polyodon (Fig. 17), for example, as well as in the whale-shark (Rhineodon), the manta 

 {Mobula) and the basking-shark (Cetorhinus), the food consists of plankton or small fish, 

 which are engulfed in the capacious mouth and swallowed whole. In all these cases the 

 gill-slits are very large but the escape of the food is stopped by an extra-branchial sieve of 

 one sort or another. At the same time the teeth are either more or less reduced {Cetor- 

 hinus, Rhineodon, Mobula) or completely absent (Polyodon). Somewhat similar adapta- 

 tions for plankton feeding are seen in some of the Clupeidae {e.g., Brevoortia). Teeth are 

 also lost in some of the protrusile jaw types. 



Protrusility. — The evolution of a strongly protrusile mouth has occurred quite inde- 

 pendently in at least two widely removed orders, the Ostariophysi (p. 189 and Fig. 76) 

 and the Acanthopterygii (p. 239 and Fig. 116). It has been noted by Delsman (1925) and 

 by Regan (1924) that in the most extreme stage among certain wrasses even the quadrate, 

 which is fixed by the surrounding bones in all preceding stages, has now acquired a flexible 

 joint with the hyomandibular and can be swung far forward to permit extreme protrusion 

 of the premaxillae (Fig. 131). In Phractolamus the toothless protractile mouth when at 

 rest can be folded back on the top of the snout (Goodrich, 1909, p. 391). 



The Branchial Arches. — As to the branchial arches themselves, obviously the most 

 important factor affecting them is the size, number and character of the gills. In the 

 primitive cephalaspid ostracoderms there were nine interbranchial septa (Stensio, 1927, 

 p. 161) or visceral arches, all bearing gills. In the existing elasmobranchs the normal 

 number of gills is five and it may well be suspected that the six or seven gill-slits of the 

 notidanoids and the six of Pliotrema represent a secondary increase in number. In all 

 ganoids and primitive teleosts there are five gills but in many of the more specialized 

 teleosts the number sinks to four and a half, four, three and one-half. In the morays and 

 gastrostomoids there has been a reduction in the size of the gills and gill-arches, which is 



