S MAEINE EEPTILES OF THE OXFORD CLAY. 



edge. The opening is, moreover, usually more or less constricted by a pair of blunt 

 processes (j}.) situated near the lower end of the opening, and it seems most likely 

 that the actual neural canal was only that portion of the opening below these processes. 

 The lower ends of the arch are occupied by gently concave triangvilar surfaces {exo.f.) 

 for union with the exoccipitals. At the upper outer angle of the occipital surface 

 there is a funnel-shaped depression, at the bottom of which is a large foramen {for.), 

 which perforates the bone, passing into the cranial cavity by a large smoothly rounded 

 aperture. The function of this opening, which does not seem to have been observed 

 elsewhere, is doubtful ; possibly it transmitted a large blood-vessel. From its position 

 it seems not unlikely that this opening may mark the line of separation between a 

 primitively separate epiotic and the true supraoccipital ; for, although Baur * has stated 

 that no trace of a separate epiotic element has ever been observed in Reptilia, this 

 seems to apply only to skulls in a comparatively advanced state of ossification, for 

 a separate epiotic has been figured by Parker f in young embryos of several types 

 (e. g. Irojjidonotus and Lacerta). Another possible explanation of this opening is, that it 

 may have given passage to a part of the enlarged upper end of the ductus endolympha- 

 ticus of the ear, such as seems to occur in some Geckoes, in which, according to 

 AViedersheim J, a portion of the enlarged saccus endolymphaticus lies on the outside of 

 the skull on and among the muscles of the neck, this external portion communicating 

 with that inside the cranium by a duct passing through a foramen between the parietal 

 and auditory capsule (or between the parietal and supraoccipital), which, it is suggested, 

 may be equivalent to the opening here noticed. Wiedersheim believes that the object 

 of this enlarged saccus, Avhich is more or less full of fine crystals, is to increase the 

 sensitiveness to sound-vibrations §. If the conjecture that some such structure existed 

 in the Ichthyosaurs is correct, it may be supposed to have compensated for the loss of 

 sensitiveness to sound-vibration that must have resulted from the peculiar modification 

 of the stapes, both as to its form, size, and relations to the surrounding bones. 



The lateral (epiotic) region of the supraoccipital projects forwards at right angles to 

 the occipital portion of the bone ; it is irregularly triangular in outline and bears on its 

 outer surface a smoothly rounded triradiate depression, which marks the position of the 

 inner wall of the anterior and posterior vertical semicircular canals [a.v.c, p.v.c). 

 These are bordered by a roughened edge of varying Avidth for cartilage, the posterior 

 edge being the broadest, the anterior of moderate width, the ventral very narrow and 

 even wanting anteriorly and posteriorly where the channels for the canals reach tlie 



• Zool. Anzeig. vol. sii. (1SS9) pp. 46^7. 



t Phil. Trans. 1S79 (pt. ii.), p. 595, pi. 41. fig. 5; also 1878 (pt. ii.), p. 385, pi. 31. figg. 3, 4. 



i Morphologiscbes Jahrbuch, vol. i. (1876) pp. 495-534, pis. xvii.-xix. 



§ Mr. Boulenger has pointed out to me that the size of this lime-containing sac varies greatly even in 

 tlili'erent individuals of the same species, and it is often apparently absent; this would, of course, be against 

 regarding this organ as having important auditory functions. 



