CHONDROCRANIUM OF SYNGNATHUS FUSCUS 437 



In Sjaignathus the cartilage forming the dorsal margin of the 

 foramen for the passage of the jugular vein and the ramus 

 hyomandibularis facialis extends more dorsally in the wall than 

 does the postorbital process (fig. 1). Posterior to the foramen 

 the cartilage forms the wall of the otic capsule proper, since the 

 membranous labyrinth appears between its wall and the brain 

 (fig. 7). The large basicapsular fenestra in the wall of the otic 

 capsule of a 5.7-mm. Gasterosteus and of a 13-mm. Sahno has 

 been interpreted as the homologue of the fenestra ovalis in the 

 otic capsule of Amphibia. This is represented in the wall of 

 the otic capsule of Syngnathus by a minute opening in its ventral 

 part, closed by membrane. 



Two cartilaginous projections from the mesial face of the 

 capsular wall, connected by fibrous tissue with the membrane 

 separating the cavum cranii from the cavum labyrinthii, are the 

 primordia of the lateral and posterior septa semicircularia. The 

 lateral septum is shown in figure 7. The posterior one is located 

 a short distance posterior to this and more dorsal. 



The dorsomesial margins of the otic capsular walls are con- 

 tinuous with a narrow median cartilage which lies in the fibrous 

 connective tissue dorsal to the brain (fig. 2). This bar of carti- 

 lage starts as a small point in the membranous roof of the cranium 

 of the postorbital region in the same transverse plane as that 

 in which the facialis foramen is located (fig. 6). It gradually 

 becomes wider posteriorly and forms a triangular plate in the 

 cranial roof (fig. 2). The posterolateral margins of this plate 

 are confluent laterally with the mesial margins of the otic capsular 

 walls. Such a plate of cartilage has been described, but not 

 figured for a 25-mm. Salmo by Gaupp, and termed the taenia 

 tectum medialis. An epiphysial bar such as is found in the 

 cranial roof of Gasterosteus connected with the ectethmoid 

 region by a pair of supraorbital cartilages is lacking in Syngna- 

 thus. Hence in Syngnathus the taenia tectum medialis is the 

 only cartilage which at this stage lies dorsal to the brain, since 

 the occipital cartilages have not as yet met mesially. It may 

 be considered as the remnant of a once solid cartilaginous synotic 

 tectum which has become very much reduced during the phylo- 

 genetic processes which gave rise to Syngnathus. 



