20 ILLINOIS BIOLOGICAL MONOGRAPHS [20 



to the condition *n the anterior region of the head of this same specimen 

 earUer in this description. 



The parachordaha do not extend beyond the anterior end of the notochord, 

 but lie lateral to and separated from it by a space filled with a stroma of pro- 

 cartilage cells. This s|pace is comparable to the posterior basicranial fenestra 

 which Gaupp has described for the 25 mm. Salmo, but which is lacking in the 

 10 mm. Amiurus. The parachordaha are triangular in cross-section and are 

 fused latero-dorsally with the floor of the saccular cavity of the otic capsule. A 

 sharp crest marks the line of division between the two, and from the dorsal edge 

 of this crest a membrane extends to the roof of the cranium, separating the 

 caviun cranii from the cavum labyrinthii. The sacculus lies on the capsular 

 side of this membrane within the otic capsule, on a higher plane than the medial 

 ends of the parachordals. As the parachordals extend posteriorly, they 

 gradually come in contact with the lateral surfaces of the notochord, at first by 

 a sharp edge which gradually becomes blunt and finally concave, as it comes 

 into closer contact with the notochord. The parachordals of the two sides 

 remain distinct from each other however, and I was unable to observe a region 

 of fusion, either above or below the notochord. The glossopharyngeal nerve 

 passes to the exterior through the cavum labyrinthii between the sacculus and 

 the posterior semicircular canal. The passage of this nerve in Amiurus follows 

 the same route between the sacculus and the semicircular canal, but the fora- 

 men lies between the dorso-lateral edge of the parachordal plate and the ventral 

 margin of the capsule, rather than in the wall of the capsule proper as it does in 

 Amia. The vagus nerve issues higher up in the wall than it does in Amiurus 

 and instead of being ventral, is posterior to the otic capsule. The cartilage 

 of the parachordals has a greater posterior extent in Amia than it has in 

 Amiurus. 



In a 25 mm. Sahno, as described by Gaupp, the parachordaha lie lateral to 

 and close around the notochord, except at its anterior tip which projects freely 

 into the posterior basicranial fenestra. This fenestra is cut off from the more 

 anterior fenestra by a transverse bar of cartilage between the anterior ends of 

 the parachordaha. The rectus eye muscles are inserted between the anterior 

 ends of the parachordaha which form the lateral walls of an eye muscle canal 

 in this region, the cavum of which is cut o£f from the cavum cranii by a mem- 

 branous floor (See Gaupp, 1906, fig. 342). Commenting upon the parachordal 

 relation Gaupp says: "Die Balkenenden verschmelzen mit den vorderen Para- 

 chordaha; die urspriingliche Grenze liegt anfangs etwa in der Hohe der vordere 

 Chordaspitze und entspricht dem (in dem Folge sich mehr verengernden) 

 Uebergang der vorderen und hinteren basikranialen Fontanelle. Schliesshch 

 tritt auch eine vordere und eine hintere Vereinigung zwischen dem Parachor- 

 dal imd dem inzwischen vergrossten periotische Knorpel ein. " 



As noted in the discussion of the otic region there is a fenestra between the 

 parachordal cartilage and the otic capsule in Salmo which is not present in 



