THE SKULL OF CHIMERA. 



Ill 



instead of curving gradually upward and forward, as in the 

 Selachii, follows the straight line of the ventral surface of the 

 brain until it reaches what Schauinsland considers to be the 

 anterior end of the cranial cavity. There the trabecular plate, 

 which at this age is still procartilaginous in its anterior portion, 

 contracts abruptly to a narrow procartilaginous bar which lies at 

 a marked angle to the trabecular plate, being directed dorso- 

 anteriorly instead of ventro-anteriorly. This narrow procarti- 

 laginous bar is called by Schauinsland the " Septum im vorderen 

 Schadelabschnitt," but it is evident that it is the strict homo- 

 logue of the rostral stalk of the Selachii. The three rostral 

 processes of Schauinsland's descriptions are all shown arising 

 from this stalk, the median process from its dorsal end and the 

 two lateral processes from its lateral edges at about the middle or 

 dorsal third of its length. A short, plate-like subethmoidal keel 

 has begun to develop on the external and hence morphologically 

 ventral surface of the stalk. 



From either side of the dorsal end of this rostral stalk, a band- 

 like process runs laterally and then posteriorly, and, spreading 

 dorsally and ventrally, fuses with the procartilaginons anterior 

 portion of the ethmoidal cartilage, which cartilage forms the 

 antorbital process and the anterior wall of the orbit. This band- 

 like process is accordingly a dorso-lateral trabecular process, 

 similar to that in the Selachii, and the large opening enclosed 

 between it, the rostral stalk, and the ethmoidal and trabecular 

 precartilages should be primarily a simple fenestra olfactoria 

 similar to that in the Selachii, but, even in the youngest embryos 

 described by Schauinsland, the dorsal portion of the fenestra has 

 apparently been already converted into a fenestra nasalis by the 

 beginning of the development of the nasal capsule. 



The development of the nasal capsule is not readily com- 

 prehended from the several figures given by Schauinsland. In 

 the figures of the 60 mm. embryo (figs. 130-131) the capsule 

 is not indicated by index-letters, but the lateral edge of the 

 anterior opening of the cranial cavity is irregular, the dorsal 

 portion projecting anteriorly beyond the ventral portion and 

 occupying the place in which, in the 85 mm. embryo (figs. 124- 

 126), the nasal capsule is shown. In an embryo said to be older 

 than the 85 mm. one, but of which the length is not given, the 

 nasal capsule is a dome-shaped protuberance of the dorso-lateral 

 portion of the cranial wall immediately posterior to the actual 

 anterior opening of the cranial cavity. Ventral to this so-called 

 capsule the lateral wall of the cranial cavity runs evenly onward 

 to the edge of the anterior opening of the cranial cavity, the 

 capsule apparently not extending into this part of the cranial 

 wall notwithstanding that the ventral portion of the nasal sac 

 must certainly lie internal to it. The nasal sac also quite 

 certainly does not project upward into the dome-shaped pro- 

 tuberance, that protuberance apparently lying above the nasal 

 sac and resembling a dome-shaped swelling that 1 find, in the 



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