THE SKULL OF CHIMERA. 



115 



Chijvlera. 



1. Neurocranium. 



In a six-month embryo of Chimcera colliei, Dean (1906, p. 108) 

 shows the trabecular projecting ventro-anteriorly at a. marked 

 angle to the parachordals, which would seem to establish that 

 when first chondrified they lay, as they do in Cdllorhynchus, 

 approximately at right angles to the parachordals. In the adult, 

 the conditions shown by Dean in this embryo still persist to a 

 marked extent, for, as shown in the accompanying figure (PI. I. 

 fig. 2), the line of the vertebral column, if prolonged, would pass 

 approximately across the dorsal edge of the postclinoid wall and 

 issue from the cranium somewhat dorsal to the base of the 

 median rostral process, the larger part of the prechordal portion 

 of the cranium projecting ventro-anteriorly below this line at an 

 angle of about 30°. 



Because of this position of the trabecule, the mid-ventral line 

 of the chondrocranium of the adult Chimcera projects ventro- 

 anteriorly, and from the level of the foramen magnum to the 

 level of the fenestra? nasales it is slightly curved, the hollow of 

 the curve presented ventro- posteriorly. Anterior to the level 

 of the fenestra 3 nasales, the mid-ventral line changes abruptly in 

 direction, running at first dorso-anteriorly and then ventro- 

 anteriorly and ending at the anterior end of the short beak-like 

 process of the chondrocranium. This latter process is morpho- 

 logically subnasal in position, as will be later explained, but as it 

 has the appearance, in lateral view, of being prenasal, it may be 

 called the prenasal process. 



Beginning at about the level of the middle of the orbit and 

 extending forward to its anterior edge, there is a median, longi- 

 tudinal, gash-like groove which lodges the degenerated tissues of 

 the extracranial portion of the' hypophysis, the groove being 

 deepest at its posterior end and gradually vanishing anteriorly. 

 This groove represents a persisting remnant of the hypophysial 

 fenestra, that fenestra being, in the adult, completely closed 

 toward the cranial cavity, as it apparently was even in the 

 chondrocranium of the sixth-month embryo shown by Dean. 

 Starting la/teral to this groove, on either side, a pronounced but 

 low and rounded ridge runs antero-latera.lly to the level of the 

 ventral end of the antorbital wall, where it turns somewhat 

 abruptly antero-ventrally and but slightly laterally and bears on 

 its end an articular facet and an articular head, the former lying 

 directly mesial to the latter and both surfaces giving articulation 

 to the mandibula. From the anterior ed^e of the articular facet 

 the narrow ventro-lateral edge of the chondrocranium runs 

 antero-mesially in a curved line, concave ventrally^ till it reaches 

 a, point slightly anterior to the ventral edge of the fenestra 

 nasalis, where it turns anteriorly and becomes the ventral edge 

 i)f the beak-like prenasal process. 



