324 HOLOSTEI 



present with rare exceptions. The preoperculum serves to bind 

 the hyomandibular, symplectic, and quadrate bones into a rigid arch 

 supporting the jaws (Figs. 360, 456). 



Whereas in the Elasmobranchii, Dipnoi, and Chondrostei the 

 otolithic masses in the auditory labyrinth are aggregates of small 

 separate nodules or crystals, in the Holostei (and also in Polypterus, 

 p. 298) they form' large solid calcareous structures. The spiracle 

 opening never persists, though a pit is found in Amia and Lepidosteus 

 (Wright [509]), which opens into the pharynx and lies near the 

 auditory capsule ; it probably represents the ' auditory' diverticulum 

 of the spiracle of Selachians [359]. 



Of these characters the arrangement of the lepidotrichia and 

 the loss of the clavicle point most clearly to an advance over the 

 Chondrostei, and seem to prove that the three next sub-orders 

 must have branched off from a common ancestor. 



The structure of the skull in the Teleostomes has already been 

 dealt with in a general way above (p. 266) ; it will be convenient 

 here to describe certain modifications of importance which occur in 

 the Holostei, and are of some taxonomic significance although they 

 may possibly have arisen independently in several groups through 

 convergence. 



In the lower fish the cranial cavity, often but very incompletely 

 filled by the brain, extends forwards between the orbits, and the 

 olfactory nerve- reaches the nasal organ directly by piercing the 

 cranial wall. In the Holocephali alone among the Chondrichthyes 

 is an interorbital septum formed between the huge orbits, and it is 

 above the brain. There is a tendency among the Holostei for the 

 brain-case to become more and more narrowed between the orbits, 

 until finally the two sides come together in the mid-line, giving rise 

 to a septum as a rule partly membranous (Fig. 304). The septum 

 forms usually from below upwards ; thus the optic foramina become 

 closely approximated, or even confluent. The narrowing in front 

 obliges the brain to retreat to the hinder ' region of the cranial 

 cavity. Now since the nasal sacs are situated in front of the 

 orbits, either the olfactory bulbs remain near them or they retreat 

 with the brain. In the first case the bulbs give off short olfactory 

 nerves and retain their connection with the prosencephalon by 

 means of greatly drawn-out olfactory tracts. In the second, more 

 usual, case the bulbs cling close to the brain and the olfactory 

 nerves are lengthened out; they are then compelled to pass on 

 either side of the very thin septum, and reach the nasal capsules in 

 front by crossing more or less freely through the orbit (Figs. 304, 

 508). Intermediate conditions are found within certain families, as, 

 for instance, in the Characinidae (Sagemehl [ 379]), where Citharinus 

 lias a moderately long* tractus and short olfactory nerves inside 



