8 ALLIS. [VOL. I. 



a spiculum of bone to the anterior clinoid process, thus forming 

 a foramen for the carotid artery, the carotico-clinoid foramen, 

 which is strikingly similar to the carotid canal of my descrip- 

 tions of Amia. The anterior and posterior clinoid processes 

 may be similarly united by a spiculum of bone, which thus 

 replaces a part of the thick glistening membrane of Amia ; and 

 the petro-sphenoidal ligament may be ossified, thus forming a 

 foramen through which the inferior petrosal sinus and the 

 sixth nerve pass (No. 24, vol. ii, pt. i, p. 43), bone thus again 

 replacing parts of the membrane of Amia. This membrane in 

 Amia accordingly deserves more attention than has heretofore 

 been given it. 



The cranial cavity of fishes is said by Sagemehl (No. 26) to 

 be filled with a mass of Fcttgcivcbc or ScJdeimgcwcbe, usually 

 voluminous, which occupies the entire space between the inner 

 surface of the skull and a single vascular membrane which is 

 closely applied to the outer surface of the brain. The outer 

 and inner surfaces of this tissue are partially differentiated 

 as limiting membranes, and between the inner of these mem- 

 branes and the vascular membrane there is a slit-like pericere- 

 bral lymph space, which Sagemehl considers as the homologue 

 of the subdural space of higher vertebrates. The single vascu- 

 lar membrane is accordingly considered by him as the pia 

 , mater and arachnoid together of higher vertebrates ; the volu- 

 minous mass of fatty or ScJilcini tissue, as the dura mater. 



The outer, partially differentiated limiting membrane of the 

 dura mater, so defined, is said by Sagemehl to be closely 

 applied to the entire inner surface of the skull, excepting only 

 the labyrinth recesses, and to be the periosteum or perichon- 

 drium, as the case may be, of the cranial cavity. It can be 

 separated into two layers, the outer of which, alone, is the 

 osteoblastic layer of the membrane, the inner layer being of a 

 fibrous character. 



In the spinal canal the outer limiting membrane of the 

 cranial dura mater seems, from Sagemehl's descriptions, to be 

 separated, as a separate membrane, from the rest of the dura 

 mater ; for he says that the spinal dura mater does not lie, as 

 the cranial dura mater does, against the inner surface of the 



