— 189 — 



has a myodome, or from those found in Polypterus which has no myodome. In Lepidosteus the 

 mesial processes of the proötics, which unite to form the proötic bridge, are said by Sagemehl to be 

 connected, across the median line, by membrane, instead of by cartilage. To me this connecting 

 tissue looks much rnore like cartilage or fibro-cartilage than like membrane, but this is unimportant, 

 for even in Amia membrane is here first formed and later chondrifies. Sagemehl further says that 

 this membrane entirely closes the canal toward the cranial cavity. This is an error, for I find the 

 anterior wall of the canal always perforated by a median oval opening through which the apparently 

 short saccus vasculosus projects. The cross-canal is filled, as Sagemehl states, with a mass of fatty 

 tissue and this tissue is richly supplied with blood. 



Immediately anterior to the cross-canal there is, in the floor of the cranial cavity, the pit- 

 like and slightly oval pituitary fossa. The lateral and anterior walls of this fossa are of cartilage. 

 The floor of the fossa is perforated by a nearly circular opening, the opening being closed ventrally 

 by the parasphenoid. Posterior to this circular opening, and extending to or slightly beyond the 

 anterior wall of the cross-canal, the floor of the pit is, in my medium-sized adults, formed of tough 

 membrane only. Posterior to this membrane the floor of the cross-canal is of cartilage, and inclines 

 gradually and slightly upward to the base of the ridge of cartilage that forms the posterior boundary 

 of the cross-canal. The circular opening in the cranial floor is accordingly the hypophysial fenestra, 

 the greatlv reduced pituitary space, or pituitary fenestra of Parker's descriptions of embryos of 

 Lepidosteus, and this fenestra of the adult Lepidosteus is the strict, but reduced homologue of the 

 hypophysial fenestra of Amia. The fenestra of Lepidosteus is said by Parker to lie between the 

 ,,roots" of the trabeculae; but, as already stated, the laterally bounding cartilages seem to me to 

 be the anterior ends of the parachordal cartilages. Parker's opinion was doubtless based; Ist., on 

 the position of the hypophysis, in early embryos, between the hind ends of the trabeculae; and, 2nd. 

 on the position of the posterior clinoid bridge of bis descriptions, which bridge is said by him (1. c. 

 p. 481) to run „straight across, joining the roots of the trabeculae together". But, this posterior 

 clinoid bridge simply represents an early stage in the development of the enclosing walls of Sage- 

 mehl's cross-canal, and hence joins regions of parachordal and not of trabecular origin. Furthermore, 

 the position of the hypophysis in early embryos need not necessarily be the same as that in the adult. 

 For, the intertrabecular and interparachordal fenestrae of early embryos being continuous, as shown 

 by Parker, the well-known unequal longitudinal growth of the cartilage of the basis cranii and the 

 overlying brain might easily pull the pituitary body backward, out of the hind end of the inter- 

 trabecular space into the interparachordal region. And this, in my opinion, has certainly taken place 

 in Lepidosteus. 



Sagemehl says that the lateral ends of the cross-canal closely approach the postero-ventral 

 corner of the orbit. This is an error, for the cross-canal is everywhere widely separated from the orbit. 

 He further says that the cross-canal exactly resembles the canalis transversus of selachians, excepting 

 that it is closed toward the orbit. This also is not wholly correct, for as the canalis transversus of 

 selachians is traversed (Allis, 'Ol) by a venous sinus and not simply by a lymph sinus, as Sagemehl 

 supposed, it must be represented, in part at least, in Lepidosteus, by the canal that transmits the 

 pituitary vein. There are here, in fact, two Spaces that would seem to be of separate and independent 

 origin; one a median space that is either primarily or secondarily related to the saccus vasculosus, and 

 the other a canal that leads from the orbit, on either side, and is traversed by the pituitary vein. The 

 saeeulaf space is roofed by the mesial processes of the proötics, and from it, or immediately in front 



