1886.] AND HYOID ARCHES IN A CKETACEOUS SHARK. 221 



gradual, but then very abrupt, while anteriorly the rise is much 

 more uniform and produces a markedly tapering outline. Quite at 

 the front, the cartilage has the appearance of being more robust than 

 is the case further back ; but this is perhaps chiefly due to the 

 infolding of the lower edge for the production of a trough for the 

 membrane bearing the undeveloped teeth. 



On comparing this form of mandibular arch with the various 

 modifications observed among living Selachians, it is at once 

 evident that none agrees so closely as that of the two genera of 

 Notidanidse. Heptanchus and Hexanchus ', indeed, exhibit an 

 arrangement that differs in no essential particular from that just 

 described in the Cretaceous Hybodont. In both cases there is not 

 only a well-developed pterygo-trabecular process — homologous (as 

 shown by Prof. Huxley -) with the pedicle of the tadpole's suspen- 

 sorium, — but also a distinct postorbital prominence and articulation, 

 corresponding to the otic process in the tadpole^. The mode of 

 articulation of the lower jaw is also nearly identical in each case; 

 and though the fossil is at present much crushed, it requires very 

 little careful study to discover that the hollows for the muscles for 

 raising the mandible were quite as deep in the Cretaceous Shark as 

 they are in the living genera under comparison ; the upper border 

 of the quadrate region, however, is much less thickened than in the 

 Notidanidai and agrees more closely with that of ordinary Selachians. 



In the hyoid arch, the upper or hyomandibular element (figs. 1, 

 2, hm, and fig. 4) is comparatively small and slender. Its length is 

 0-037 m., and the cartilage is considerably arched and flattened 

 in what appears to have been an antero-posterior direction. The 

 proximal extremity is imperfect, but was evidently somewhat 

 expanded at its articulation with the cranium ; this end is also 

 slightly twisted with respect to the axis of the rest of the element. 

 Just below the bend, the cartilage appears contracted a little when 

 viewed from behind, but soon expands again, forming a blunt 

 tuberosity {t) on the side nearest the pterygo-quadrate ; and from 

 this point it finally becomes gradually narrowed until its termmation 

 in the imperfectly-displayed articulation for the cerato-hyal. 



The cerato-hy'al (figs. 1,2, ch) is 0-048 m. in length, and is com- 

 pletely shov.n on the left side of the fossil, though somewhat mutilated 

 at the distal end ; the lower jjart, however, is well preserved on the 

 right. The cartilnge is considerably arched in the ordinary manner, 

 and is much less robust towards its upper end than in the rest of its 

 length. Compared with the hyomandibular, it is remarkably stout. 

 A little below the proximal end it becomes comparatively large and 



^ See figures by C. Gegenbaur, '' Uutersuchimgen zur vevgleichenden 

 Auatomie der Wirbeltbiere.— III. Has Kopt'skelet der tselachier," pi. x. J am 

 also indebted to tbe kindness of Mr. Howes iiud Mr. Martin Woodward for 

 every facility for studying the beautiful prepar.itions i)i Hepfanchus, Cesfrac/oii, 

 &c. in tbe Biological Laboratory of tbe Normal School of Science. 



- T. H. Huxley, loc. cif. p. 40. 



^ It is interesting to note that Prof. Cope's Perniia'i Selachian skulls already 

 referred to also exhibit this character. 



