ON THE SKULL OF CHI M /ERA. 



105 



PAPERS. 



7. The Prechordal Portion of the Choudrocranium of 

 Chimcera collieL By Edward Phelps Allis, Jiul, 

 F.Z.S. 



[Received December 27, 1916; Read March 20, 1917.] 

 (Plates I.-III *) 

 Index. 



Page 



S tructure or Morphology . ..... 105 



The prechordal portion of the chondrocranium of the adult 

 Chimcera is directed ventro-anteriorly, and not anteriorly as 

 Hubrecht's (1877) figure of this fish would lead one to suppose. 

 This is unquestionably due to the trabecular having been first 

 laid down at a marked angle to the parachordals, and to their 

 having retained, to a marked extent, that primitive position, 

 instead of, as in the Selachii, later gradually acquiring a position 

 approximately in the line prolonged of the parachordals. The 

 chondrocranium of Chimcera at these early stages has unfor- 

 tunately not been described. It has, however, been described at 

 these stages in certain of the Selachii, and at slightly later stages 

 in Callorhynchus, and reference must be made to the conditions 

 in these embryos of these fishes before describing those in the 

 adult Chimcera. 



Selachii. 



In Acanthias and Pristiurus, and hence probably in all of the 

 Selachii, the cranial flexure is so great when the neurocraniuui 

 begins to chondrify that, as shown in SewertzofT's (1899) figures 

 of embryos of these fishes, the parachordals and the enclosed 

 notochord project slightly into the hollow of the mesocephalic 

 flexure (plica encephali ventralis, von Kupfier, 1906). The 

 trabecular cannot, accordingly, be laid down in the line pro- 

 longed of the parachordals, and, preserving their normal relations 

 to the ventral surface of the brain, they at first lie at right 

 angles to the parachordals and slightly posterior to their anterior 

 ends. The marked cranial flexure at this stage also affects the 

 position of all the visceral arches that lie anterior to the hyal 

 arch, and the palatoquadrate and mandibula, which represent 

 either the cartilaginous bar of the mandibular arch alone, or 

 that bar together with parts of the bars of one or more pre- 

 mandibular arches, lie, when first laid down, at right angles to 

 the trabecular, and hence in a nearly horizontal instead of a 



* For explanation of the Plates see p. 143. 



Proc. Zool. Soc— 1917, No. VIII. 8 



