The Embryology of Heterodontus japonicus 699 



On the basis of the mode of suspension of the jaws, it appears impossible to make 

 a clear-cut distinction between the families Hybodontidae and Cestraciontidae as consti- 

 tuted by Woodward (in Zittel, 1932). Some genera of the Hybodontidae (e.g., Orodus) 

 are known only by their teeth, or by their teeth and dorsal spines. Where genera are 

 represented by fairly complete skeletons (e.g., as in Hyhodus), there is apparently some 

 lack of uniformity in the method of jaw suspension. Nevertheless Woodward (1921) 

 generalized as follows: ''The Hybodonts. . . are especially interesting because, while 

 their dentition and their general appearance resemble those of the existing Cestraciont- 

 idae, their skull is very different and more closely agrees with that of the Notidanidae". 

 It is possible that the word skull, as used here, means cranium, as it seems to do in 

 several places in Woodward's writings. 



The terms autostylic, hyostylic and amphistylic were introduced by Huxley (1876) 

 to designate three types of skull and of suspension of the first visceral arch — the mandibu- 

 lar arch, or the jaws. We are here concerned only with the second and third types as 

 they occur in sharks. In both, the palatoquadrate cartilage (constituting the framework of 

 the upper jaw) is quite distinct from the chondrocranium. The palatoquadrate is, at 

 most, in contact with the cranium only by articular surfaces, and connected with it by 

 ligaments. In front, the palatoquadrate is often loosely connected with the lateral 

 ethmoid (preorbital) region of the skull by way of a palatobasal or ethmoid process (of the 

 palatoquadrate), but this type of connection apparently has little or nothing to do with 

 the classification under consideration. In most sharks, the dorsal element of the hyoid 

 arch, called the hyomandibular cartilage, attains a large size, gains an attachment to the 

 auditory capsule, and becomes the chief apparatus for suspending the palatoquadrate from 

 the cranium. This type of suspension is called hyostylic, and is exemplified by the skull of 

 Scyllium (Text-figure 32). In the hyostylic skull the upper jaw is held somewhat away 

 from the cranium, and retains a considerable degree of mobility. In the amphistylic 

 skull, according to Huxley, the palatoquadrate cartilage is wholly, or almost wholly, 

 suspended by its own ligaments; the hyomandibular is small and contributes but little to 

 its support. Some authors (e.g., Goodrich, 1909, p. 95) have interpreted, or modified, 

 this definition to require that, in the typical amphistylic skull, the quadrate region of the 

 upper jaw must have a postorbital articulation with the auditory capsule in addition to 

 being connected with it by the hyomandibular: as in Heptanchus (Goodrich, 1909, Fig. 

 59a) ; a typical Acanthodian (Goodrich, 1909, Fig. 159); and in Hyhodus hauffia^ius accord- 

 ing to Jaekel (my Text-figure 28) . 



It will suffice here to attempt a comparison between the skulls of Heterodontus and 

 Hyhodus, with special reference to the manner in which the jaws are attached to the 

 cranium. The skull of Heterodontus (Text-figure 33) is usually classed as hyostylic, 

 though it does not conform closely to this type. One should examine also the more 

 elaborate figures of the skull of Heterodontus philUpi by Huxley (1876, Fig. 8) and that of 

 H. francisci by Daniel (1915, Fig. 6, pi. IV). In both figures the cranium is more closely 

 molded on the palatoquadrate cartilages (upper jaws) than is represented in Goodrich's 



