THE SKULL OF CHIM.I^RA. 109 



the nasal sac, is shown, slightly developetl, in Parker's (1876) 

 figures of embryos of Scijlliitr)i, the process there projecting 

 laterally from the ventral ei[ge of the subethmoidal keel. This 

 process is called by Parker the cornu trabeculae ; but two other 

 cartilages on each side of tlie head are also so designated by him, 

 one being the cartilage c of Sewertzofl[;''s descriptions, and the 

 other the lateral bar of the rostral basket. This ventro-lateral 

 trabecular process is not shown or described by Sewertzoff even 

 in the oldest of his embryos, but that it is not peculiar to fishes 

 in which there is a subethmoidal keel is shown by the conditions 

 in the adult Chlaimjdoselachas, where the process is found well 

 developed and there forming the anterior portion of the solum 

 nasi of my descriptions of that fish (Allis, 1913). 



The beginnings of a process that lies postero-ventral to the 

 nasal sac is shown, in Sewertzoft's figures, at the ventro-lateral 

 corner of the ethmoidal cartilage, and it is apparently it alone 

 that is later prolonged into the important ventro-latero-posteriorly 

 directed process that is currently called the antorbital or ethmo- 

 palatine process. This antorbital process is, however, said to be 

 formed, in certain other fishes and in higher vertebrates, by an 

 outgrowth of the trabecular cartilage. It is therefore possible 

 that there are two processes here, one of ethmoidal and the other 

 of trabecular origin. If so, they are indistinguishably fused 

 with each other in the Selachii, and to avoid confusion I shall 

 call the process, whatever its origin, the ventro-lateral antorbital 

 process, the entire antorbital process being considered to form 

 the anterior wall of the orbit and to include both this ventro- 

 lateral process and a more or less developed dorso-lateral 

 process found in the adults of most fishes and frequently called 

 the antorbital or preorbital process. 



The sphenolateral cartilage has, in the oldest embryos of 

 Acanthias described by Sewertzofi", grown forward and fused with 

 the ethmoidal cartilage, and its supraorbital ridge, turning 

 downward at its anterior end, forms the dorsal portion of the 

 lateral edge of the antorbital process, that portion of that edge 

 corresponding to the well-developed dorsolateral antorbital 

 process found in certain other fishes. The ramus ophthalmicus 

 superficialis trigemini and the nervus ophthalmicus profundus 

 both, in younger embryos, run forward through the orbit and 

 then dorsal to the nasal sac, passing external to the ethmoidal 

 cartilage, between the dorso-lateral and ventro-lateral antorbital 

 processes. But when the sphenolateral and ethmoidal cartilages 

 fuse, the two nerves and an accompanying vein and artery 

 become more or less enclosed between them, either separate!}^ or 

 together. The nasal vein, which comes fi'om the nasal pit to fall 

 into the orbital venous sinus, also either becomes enclosed 

 between these two cartilages or is enveloped in the latei-al edge 

 of the ethmoidal cartilage alone. 



The nasal sac of embryos of the iSelachii thus lies between four 

 proce.sses, more or less developed, two being derived from the 



