1906.] THE SKULL OF A YOUNG RIBBON-FISH. 553 



Bones in the Orbit. 



In front of the sphenotic are two pairs of bones contributing 

 to the roof of the orbit — the alisphenoids and orbito-sphenoids 

 (PL XXXIX. figs. 5, 6). 



The Alisphenoid is a flat bone not quite reaching to the outer 

 edge of the orbit. Between and somewhat behind the two 

 alisphenoids is a large triangular foramen which Parker discusses 

 at length. He mentions that the only diiEculty in the way of 

 the interpretation that these bones are alisphenoids is, that 

 there is no foramen for the fifth nerve behind them ; but he 

 points out that in many mammals the first division of the fifth 

 nerve passes out altogether in front of the aUsphenoid. In the 

 Rabbit, too, the first two divisions of the fifth nerve pass out of 

 the skull by the sphenotic fissure, which lies between the ali- 

 sphenoid and the basisphenoid. This is relatively in the same 

 position as the foramen in the skull of R. parheH. In front of 

 the alisphenoid is a large bone extending as far forward as the 

 mesethmoid. This Parker identified as the orhito-sphenoid. 

 There is no suture in the middle line, so that the two bones must 

 be hei'e fused. The oi'bito-sphenoid is not perforated by the 

 second nerve, which must pass out through the foramen between 

 the alisphenoids. 



Cranial Hoof. 



The roof of the cranial cavity is formed by the frontal and the 

 pai'ietal bones along with the median plate of cartilage, the 

 " tegmen cranii" (PI. XXXIX. figs. 4, 6). 



The Frontal is a large bone, relatively much larger than in 

 E. glesne, extending as far forwards as the ectethmoid and 

 backwards nearly to the posterior end of the supraoccipital. 

 Thus they form the greater part of the upper surface of the skull 

 and the lateral margin of the anterior dorsal groove for almost its 

 whole extent. Further, each sends back a small process on the 

 mesial side of the parietal, which thus only just reaches the 

 margin of the posterior dorsal groove by a corner. The fi'ontals 

 do not, as is usual in Teleosts, meet in the middle line, as Parker 

 pointed ou^t for E. glesne, but are separated by the "tegmen 

 ci-anii." Posteriorly, the frontal meets the parietal and pterotic, 

 and sends a process back under the parietals which nearly meets 

 the epiotic. On the under surface of the skull the frontal is 

 seen to form a supraorbital plate (PI. XXXIX. fig. 5). 



The Parietal is a small bone, very different in form and size 

 from that of JR. glesne ; it is long and narrow, extending forwards 

 between the frontal and the " tegmen cranii," and forming a 

 rounded prominence at the side of the supraoccipital. In the 

 present species, although the boundaries of the bone were very 

 difiicult to determine owing to its thinness and friability, it seems 

 to have a very difierent form. The internal backward process 

 of the frontal almost cuts ofF the parietal fi'om participation in 

 forming the side of the dorsal groove, and far fi'om forining a 



