— 188 — 



A short leg of the alisphenoid forms, in the ädult, the anterior boundary of the oculomotorius 

 foramen, the ventral end of the leg being continuous with the small flat bar of cartilage that lies 

 between the ventral portion of the optic fenestra and the foramen for the pituitary vein; and as this 

 bar of cartilage rises from the lateral edge of the basisphenoid region of the trabecular cartilage, the 

 associated leg of the alisphenoid must be the basisphenoid leg of that bone. Of the parasphenoid 

 leg of the alisphenoid I can find no trace, the alisphenoid of Lepidosteus thus being strictly of the 

 usual teleostean type. 



The recti muscles of the eye all have their origins from the skull immediately ventral and 

 posterior to the optic fenestra, and hence in the immediate neighbourhood of the foramen for the 

 pituitary vein. The rectus externus, in the 80 mm specimen, separates into two portions as it ap- 

 proaches the skull, and the tendon of one of these portions traverses the foramen with the vein, and, 

 following it, has its origin beneath it; but whether its point of origin is on the cartilage of thebasis 

 cranii, on the parasphenoid, or in fibrous tissue of the region, I could not determine. 



Although wholly unrelated to the present subject, it may here be stated that the rectus in- 

 ternus arises, in Lepidosteus, by a long and slender tendon which is closely applied to the antero- 

 mesial surface of the rectus inferior, these two muscles thus arising as a single nvuscle and being, 

 practically, not yet fully differentiated parts of a single muscle; this being in accord with my suppos- 

 ition (97a) that these two muscles of teleosts and bony ganoids arise by the Splitting of the single 

 rectus inferior of elasmobranchs. These two muscles, and the rectus superior and obliquus inferior, 

 are innervated by the oculomotorius in the manner and order that they are in Amia, this confirming 

 my Interpretation ('97a, p. 520) of Van Wijhe's description of this fish. The abducens leaves the 

 cranial cavity through the trigemino-facialis foramen, piercing the membrane that closes that foramen 

 antero-ventral to and independent of the trigemino-facialis roots. It then runs antero-laterally, vent- 

 ral to the jugular vein, between it and the external carotid, lying internal and antero-internal to 

 the ventral edge of the trigemino-facialis ganglion, and reaches the rectus externus. It does not 

 pass over the truncus trigeminus, as stated by Schneider ('81), but, as the rectus externus passes 

 over that truncus, the abducens naturally also would, if it were sufficiently prolonged. The radix 

 profundi, after its exit from the skull, enters an extra-cranial profundus ganglion which lies close 

 against the side wall of the skull immediately dorso external to and in contact with the oculomo- 

 torius, and not below that nerve, as shown by Schneider. Two ventral or ventro-anterior prolong- 

 ations of the ganglion terminate in small ganglionic swellings, which lie in contact with the inferior 

 branch of the oculomotorius and are the ciliary ganglia. From the anterior end of the profundus 

 ganglion a stout portio ophthalmici profundi arises. The trigemino-facialis ganglion lies, as in Amia, 

 wholly outside the cavum cranii, a few scattered cells only being found in the roots of the nerves. 



The cross-canal of Sagemehl's descriptions can now be considered. The enclosing walls of 

 this canal form, in the fresh skull of the adult fish, a prominent transverse bolster on the floor of 

 the cranial cavity, and this bolster must quite unquestionably have arisen by the depression of the 

 anterior edge of the proötic bridge of a fish like Amia (Allis, '97a, Fig. 11) or Polypterus (Pollard, 

 '92, Fig. 12) until it met and then fused with the floor of the cranial cavity on either side of the 

 Saccus vasculosus. The cross-canal of Lepidosteus is thus a strictly intramural space that 

 lies beneath a proötic bridge of the primary type found in Amia, and that is related either primarily 

 or secondarily to the saccus vasculosus. This space represents the posterior half only of the myo- 

 dome of Amia, and it might have been developed either from the conditions found in that fish, which 



