21] THE SKULL OF AMIURUS— KINDRED 21 



Amiurus, and the ninth nerve issues through its posterior end. Whether the 

 foramen for the passage of the ninth nerve in Amia and Amiurus is a remnant 

 of this or not, it is hard to say without knowing the earlier history. The saccu- 

 lar relations are probably the same in Salmo as they are in Amia, as the Siluri- 

 dae have a specialization not found in all of the teleosts, just as the eye muscle 

 relations are peculiar in a certain large group with well-developed eyes. 



The relations of the anterior end of the parachordals of Amiurus and of 

 Salmo are homologous in that they lie between the otic capsules on either side 

 of the notochord and are connected anteriorly with the trabeculae. Primitively 

 they are alike, but specialization in one form in connexion with the ear and in 

 the other with the eye have made detailed comparisons difficult. 



The occipital region. As remarked above, the dorsal part of the occipital 

 arch forms the posterior margin of the posterior fontanelle (Fig. 2). The 

 occipital-otic capsule fusion takes place above the foramen for the vagus nerve, 

 and behind it the ventral ends of the occipital arch are fused for a short distance 

 to the parachordals (Fig. 21). Behind this region of fusion the anterior ends 

 of the scaphoid processes project between the occipital arch and the parachor- 

 dals. At this stage, the scaphoid process is a membranous plate connected 

 posteriorly with the perichondrium of the cartilaginous scaphium (Fig. 13). 



The first post- vagal nerve (Figs. 17, 35) leaves the vertebral canal between 

 the anterior end of the scaphoid process and the ventral end of the occipital 

 arch. This part of the arch is enclosed in a perichondrial ossification, even at 

 this stage. The anterior margin of the foramen for the passage of the nerve 

 is formed by the occipital arch-parachordal fusion. The cartilage of the para- 

 chordals does not extend posterior to this immediate region, and the diameter 

 of the notochord is much larger than it was intercranially (compare Figs. 8 

 and 17). The elastica interna and externa are -^ery distinct from each other 

 in this part of the notochord. 



On the median dorsal surface of the notochord there is a thickened mass 

 of connective tissue, the endorhachis, which forms a floor for the support of 

 the spinal cord (Fig. 13). This floor is supported laterally by connexion with 

 the ventral ends of the occipital arch. In this manner the space between the 

 occipital arch and the notochord is divided into three chambers, a dorsal un- 

 paired one containing the spinal cord, and two lateroventral chambers, the 

 lateral walls of which are formed by the scaphoid processes. The saccus 

 paravertebralis hes external to each scaphoid process and contains the ossicles 

 of the Weberian apparatus. The lateral chambers within the scaphoid pro- 

 cesses are called the atria sinus imparls and are the posterior continuation of 

 the sinus impar. 



The scaphiimi (Fig. 13) at this stage, is a small piece of cartilage which 

 articulates with the dorso-lateral surface of the notochord by a rounded end. 

 It has all the appearances of a modified neurapophysis. Between its dorsal 



