MORPHOLOGY OF TELBOSTEAN HEAD SKELETON. 56*? 



Polyodoii (Bridge, 78) retains this last feature, together with 

 " Praefontallucke " and well-developed parietal foramina, 

 representing post-cranial fontauelle, into the adult. 



All definite reference to the orbito-sphenoid has been 

 purposely omitted hitherto, but it should be remembered 

 that whilst Allis states that it is a cartilage bone in Amia 

 (97, p. 7), both Vrolik (73, p. 57) and Parker (72, p. 106) 

 look upon it as a distinct membrane bone; if the latter are 

 right, then to the already existing optic fontanelle must be 

 added the space occupied by this bone. 



According to Sewertzoff (99) young Acanthias possesses a 

 large dorsal fontanelle, a lai-ge lateral one transmitting at 

 first trigeminal and facial as well as optic nerves, and Avidely 

 open auditory capsules. In Raia (Parker, 76, p. 216) the 

 dorsal fontanelle is divided by a transverse '' cartilaginous 

 beam." 



Thus the development of Acanthias, as well as the adult 

 anatomy and comparative ontogeny of Teleostomes, leads 

 back to an ancestor with a fenestrated cranium. 



In the light of all this it is interesting to read Parker's 

 account of this region in the Marsipobranchs (83), especially 

 in a moderately young lamprey. After mentioning the 

 presence of a large pituitary fontanelle, he goes on to say 

 the "side^ walls of the chondrocranium of the lamprey are 

 well developed," but " optic and trigeminal nerves pass out 

 of considerable fenestra?, and not out of mere foramina." 

 " The orbito-sphenoidal region is wider than the alisphenoidal, 

 but the 1-atter mounts up into the roof, and the two sides 

 meet round the middle and fore-part of the hind brain. The 

 occipital ring does not exist" (p. 414). He describes the 

 presence in the front of this posterior sphenoidal "tegmen " 

 of a " large pyriform fontanelle," and also states that the 

 passage from the auditory capsule into the cavum crauii is 

 large, especially in Myxine. 



Apart from the absence of occipital region and of ossifica- 

 tion, all that is required to complete the broad resemblance 

 ' The italics are not mine. 



