TUF, SKULL Ol' CHfM KHA. 115 



ClIIM.KRA. 



1 . Xcurocraninm. 



In a six-month embryo of (.'himcera coUiei, Denn (1906, p. 108) 

 shows the trabeculae projecting ventro-antei'iorly at a, marked 

 angle to the parachordals, Avhich would seem to establish tliat 

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

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

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

 mai'ked extent, foi', as shown in the accompany in,g figui'e (PI. I. 

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

 approximately across the doi'sal edge of the postclinoid wall and 

 issue from the cranium somewhat dorsal to the base of the 

 median I'ostral process, the larger part of the prechordal poi'tion 

 of the cranium projecting venti'O-antei'iorlj' below this line at an 

 angle of about 30"^. 



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

 of the chondrocranium of the adult Chlwzera projects ventro- 

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

 level of the fenestrfe nasales it is sliglitly curved, the hollow of 

 tlie curve presented ventro-posteriorly. Anterior to the level 

 of the fenestrse 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 chondi-ocranium. 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 

 tlie extracranial portion ' of the hypophysis, the groove being 

 deepest at its posterior end a.nd gi-arlually vanishing anteriorly. 

 This groove represents a persisting remnant of the hypophysial 

 fenesti-a, that fenestra being, in the adult, completely closed 

 towai'd the cranial cavity, as it apparently was even in the 

 ehoirdrocranium of the sixth -month embryo shown by Dean. 

 .Starting latei'al to this groove, on either side, a pronounced but 

 low and rounded ridge runs antero-laterally to the level of the 

 ventral end of the antorbital wall, where it turns somewhat 

 abruptly antei-o-ventrally and but slightly laterally and bears on 

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

 directly mesial to the latter and both surfaces giving articulation 

 to the mandibula. From the anterior edge 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 

 of the beak-like prenasal process. 



