ON CERTAIN CROSSOPTERYGIANS. 1247 



This canal was considered by me in 1918 as the trochlearis canal 

 (Stensio, 1918 &), a view which must be incorrect in the light of 

 the facts now known. 



At the anterior end of the sphenoid, in the suture between 

 this and the exethmoideo-preethmoid, a rather wide canal {c7h, 

 text-fig. 1 ; PI. I. figs. 1, 2) is found which leads forwards to the 

 olfactorius canal, and which vindoubtedly has contained vessels 

 to and from the nasal pit {cf. Stensio, 1922 h). Perhaps it may 

 also have been in some relation to the points of origin of the 

 musculi obliqui of the eye. 



The arteria cai-otis externa must, so far as one can understand, 

 already have been sent out from the artei-ia carotis communis 

 behind the basipterygoid process, and has probably run upward 

 approximately as far as to the facialis exit, then turning forward 

 and continuing dorsally of the basipteiygoid process to the orbit. 

 The arteria carotis interna, after its origin behind the basi- 

 pterygoid process, must have run forward ventrally to this 

 process and laterally to the parasphenoid to its ascending canal 

 described above in the sphenoid. The position of the ventral 

 opening of the ascending canal in relation to the nerve exits is 

 seen from text-fig. 1 and PI. I. figs. 1, 2, 4. The vena jugularis, 

 after arriving at the dorsal end of the basipterygoid process, 

 must have passed backward dorsally of this process, as already 

 pointed out, then continuing backwards between the trigeminus 

 and facialis exits. 



There is evidently no trigemino-facialis chamber and no 

 myodome in the fish. According to the course of the nerves and 

 vessels, the basipterygoid process with its antero-ventral parts 

 undoubtedly corresponds to certain posterior parts of the basi- 

 pterygoid process in Birgeria, with its dorsal parts on the other 

 hand, corresponding to the basipterygoid process of the Coela- 

 canthids {cf. Stensio, 1922 a, pp. 205-206; 1922 6; cf. also 

 pp. 1259-1266 below in the present paper). 



The ethmoidal region rapidly decreases in height forw^ards, 

 becoming very low at the anterior end. It is mainly occupied 

 by the paired exethmoideo-preethmoid, only certain smaller 

 anterior and dorsal parts being cartilaginous. At the postero- 

 medial part the exethmoideo-preethmoid is separated from 

 the sphenoid by a short synchondrosis, as already pointed out 

 above. In the posterior two-thirds of the length its plane 

 ventral surface is covered by the parasphenoid. Its lateral 

 surface is accessible for investiga,tion only in the anterior parts, 

 being otherwise concealed by remains of very fractured dermal 

 bones. 



As already mentioned, the exethmoideo-preethmoid is traversed 

 by the rather wide ca.nal for the n. olfactorius (I, text-fig. 1 ; 

 PI. I. figs. 1, 2), which runs forwards, downwards, and laterall}'. 

 The large anterior opening of this canal is incompletely sub- 

 divided into a postero-dorsal division situated at the anterior end 

 of the lateral surface of the reeion and an antero-medial division 



