IO ALLIS. [VOL. I. 



the orbits as well as elsewhere, by membrane (No. 27, p. 86). 

 In this closed canal of Lepidosteus, which is considered by 

 Sagemehl, as it unquestionably is, as the homologue of the 

 apparently open eye-muscle canal of Amia, fatty tissue is found, 

 similar, undoubtedly, to the fatty tissue found both in the ap- 

 parently open canal and in the cranial cavity proper of Amia. 



The eye-muscle canal in Amia and Lepidosteus, and hence 

 probably in all fishes, is thus an intracranial space, opened 

 secondarily toward the orbits, Lepidosteus presenting the 

 primary condition of the canal in all fishes, and not a secondary 

 one, as Sagemehl was led to conclude. It is also, according to 

 Sagemehl's definitions of the cranial membranes, an intradural 

 space, notwithstanding his indirect statement that it is extra- 

 dural (No. 27, p. 87); and as it gives passage, in Amia, to 

 certain nerves and arterial and venous vessels and lodges the 

 Gasserian ganglion (No. 2), it is a space similar to, if not 

 strictly homologous to, the cerebral sinuses and the cavum 

 Meckelii of human anatomy. It is, however, in Amia, a space 

 that certainly lies morphologically in, and not internal to, the 

 membranous bounding walls of the primordial skull. It must 

 accordingly lie in or external to, and not internal to, the cra- 

 nial homologue of those extradural fibrous tissues of Sage- 

 mehl's descriptions that line the inner surface of the spinal 

 canal of fishes. This is all sufficiently evident from Parker's 

 statement (No. 20, p. 131) that in salmon larvae the mem- 

 branous, interorbital cranial walls, later ossified as the orbito- 

 sphenoids, "pass down into the interorbital septum, which is 

 continuous below the perichondrium of the tilted, and coalesced 

 trabeculae"; and Studnicka's statement (No. 30, p. 618) that 

 the membranous brain capsule of Ammocoetes " sich dorsal an 

 die Trabeculae ansetzt." 



The fatty tissue found in the eye-muscle canal, both in 

 Amia and Lepidosteus, is then presumably similar, in general 

 character, to that found in the membranous brain capsule of 

 the Cyclostomata (No. 30), and naturally subject to chondrofi- 

 cation, as in those fishes, and hence to subsequent or indepen- 

 dent ossification. Such being the case the eye-muscle canal 

 in different fishes, and the corresponding space in other verte- 



