HOLOCEPHALI 169 



The lateral-line system is very well developed (Garman [148, 

 149], Collinge [90]). As a rule, the canal is an incompletely closed 

 or a widely open groove, and its wall is strengthened by crescentic 

 ossicles or small plates, probably modified denticles (Fig. 135, B). 

 Deeply embedded ampullary organs are present on the head. 



A peculiar rostrum supported by cartilage processes is developed 

 -on the snout. In living genera the nostrils are found to be ventral 

 with grooves leading to the mouth, as in many Selachii. The lips 

 Are strengthened with large complex labial cartilages (Fig. 134). 

 The gape of the mouth is small (Fig. 144). 



The eyes become very large, and the orbits tend to come close 

 together in the middle line. The membranous side-walls of the 

 brain-case, near each other in Callorhynchus, in Chimaera form an 

 interorbital septum above the brain, which throws backwards and 

 downwards the diminished brain- cavity (Fig. 135). Thus the 

 brain lies below the septum. The branchial slits are reduced, in 

 modern forms, to four, and are covered over by a membranous 

 operculum borne by the hyoid arch and its well-developed branchial 

 rays. The gill-slits and arches are drawn close together as in the 

 higher operculate fish, and the septum between the gill-lamellae 

 is considerably reduced as in the Dipnoi (Fig. 57), no doubt owing 

 ;to the development of the operculum. The spiracular slit has 

 been lost. There is no spiracular gill, but a posterior hyomandibular 

 ihemibranch, three holobranchs, and an anterior hemibranch on the 

 fourth branchial arch. 



The whole structure of the jaws and skull has been profoundly 

 modified in connection with the development of permanent paired 

 grinding plates. There are generally a small anterior ' vomerine ' 

 plate and a posterior large ' palatine ' plate on the upper jaw, and a 

 .corresponding large plate on the lower jaw (Fig. 141) (Jaekel [241], 

 Dean [llOa, etc.]). These plates are generally differentiated into 

 An extensive flattened basal region, and more prominent grinding 

 ridges or tritoral areas ; the former of trabecular or vasodentine, 

 the latter with a covering of vitrodentine (Fig. 141). They have 

 persistently growing bases, are not shed and replaced, and in the 

 embryo show no distinct signs of having been formed by the 

 fusion of separate denticles. Yet they must be considered as the 

 modified derivatives of the teeth of the upper and lower jaws, 

 and the tubercles often present on the ' tritors ' may represent 

 originally separate elements. To support the grinding plates 

 the jaws are strengthened and shortened. The two rami of the 

 lower jaw are fused in front, while the palato-quadrate cartilage 

 is fused to the ethmoid region of the skull in front and to the 

 auditory capsule behind (Fig. 135) (Huxley [230], Dean [IKk], 

 Schauinsland [383]). The branchial arches are normal ; the 

 hyoid arch, however, shows no large hyomandibular element, but 



