440 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



(Fig. 94) the minute eyes and entire neurocranium have been pushed forward to near the 

 tip of the snout, so that the neurocranium is remarkably short and wide. 



Influence of the Auditory Capsules 



The auditory capsule with its various subdivisions is naturally a potent element in 

 moulding the plastic osseous tissue of the cranial vault. In many fish a "bulla" is formed 

 by the prootic and basioccipital for the reception of the sagitta, or main otolith. The 

 latter in turn varies considerably in size. In the deep-sea isospondyl Opisthoproctus 

 (Fig. 43) it forms a very large, more or less circular disc. Similarly the semicircular canals 

 and ampullae require corresponding canals and swellings in the osseous cast surrounding 

 them. As these structures therefore subtract from the area available for support in a 

 region subject to severe stresses, the remaining skeletal parts are built up into more or less 

 dense or protruding pillars, in accordance with the principles already mentioned. 



Adjustments in the Ethmoid Region 



The variations in the olfactory capsules in sharks play an important part in modifying 

 the fore part of the skull, as we see in an extreme form in the hammer-head; but even in the 

 teleosts, with their usually great reduction of the olfactory capsule, it must be protected by 

 the nasals and by the lateral ethmoid, which is usually perforated by the olfactory nerve. 

 Hence stability of the lateral ethmoids is necessary and so we find them securely tied in 

 place by the surrounding parts. Moreover the ethmo-vomer block must ordinarily serve 

 as the keystone for the articulation of the opposite palatines, maxillae, premaxillae; hence 

 either the lateral ethmoids or the mesethmoids must be stoutly built and variously modified 

 to suit each case. In the gurnards (Prionotus), for example, the mesethmoid forms a large 

 median plate at the front end of the cranial shield; in Callionymus and Drepane the meseth- 

 moid is displaced behind the enlarged lateral ethmoids (Starks 1926a). It seems not un- 

 likely that further study of the ethmo-vomer block from a combined functional and phylo- 

 genetic viewpoint may permit further utilization and helpful interpretation of the accurate 

 topographic data recorded by Starks. 



Where the bony rostrum is greatly produced in widely different families, as in the 

 swordfish, scombresocids, Protosphyrana, etc., it is natural to expect the variable composi- 

 tion of the rostrum and different ways of strengthening its junction with the interorbital 

 bridge and palatopterygoid arch. 



Effects of Special Accessory Organs (Trigger, Illicium, etc.) 



It seems hardly necessary to state that various special organs such as the "trigger" 

 of the balistids, the illicium of the ceratioids and the sucking-disc of the echeneids, have 

 each brought about special modifications of the ethmo-vomer block, which are readily 

 recognizable in every case so far noted and which largely account for the decidedly queer 

 characters found in the ethmoid regions of these forms. 



The development of backwardly directed spikes on top of the neurocranium as well 

 as on the preoperculars, operculars and elsewhere is especially characteristic of the scor- 

 paenoids. The initiative in this instance perhaps comes from the integument rather than 

 from the underlying bone, which responds to it by developing a concentration of bony 

 tissue for the support of the spike. 



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