96 ALUS. [Vol. II. 



nerves, but in older ones is usually represented by a ventral 

 root only. The third segment lies between the hind end of the 

 skull and the first free vertebra, and the nerve related to it is 

 said to be the first spinal nerve. This nerve, however, leaves 

 the vertebral canal with the hypoglossus, " durch eine einzige 

 Oeffnung zwischen dem Occipitale und dem ersten Wirbel," 

 What this opening may be in or through is not indicated, but 

 the fact that the hypoglossus traverses it warrants the supposi- 

 tion that, in the adult, it must be enclosed in the hind end of 

 the skull. The post-occipital nerve of Salmo thus probably 

 agrees, in this respect, with the corresponding nerve in Scomber. 

 Young larvae of Salmo also agree with Scomber in the number 

 of occipital muscle-segments, but there is, in Salmo, one less 

 spino-occipital nerve than in Scomber. 



In Necturus, Piatt (No. 4) says that the first postotic somite 

 aborts and disappears without giving rise to muscle fibers, and 

 that this is true also for all other vertebrates above the Selachii 

 of which she knows. If it be assumed that Amia agrees in this 

 with Necturus, the nerve of the fifth muscle-segment is seen to 

 be, in both these animals, the anterior nerve of the brachial 

 plexus. The dorsal arch next posterior to this nerve is, in 

 Amia, the posterior occipital arch. In Necturus it is the arch 

 of the third free vertebra. The skull of Amia would thus, 

 under this assumption, contain three vertebrae found free in 

 Necturus. If, on the contrary, it be assumed that the first 

 postotic somite of Amia does not disappear, but gives origin to 

 muscle fibers, the two occipital arches of Amia would correspond 

 to the dorsal arches of the first two free vertebrae of Necturus, 

 as they do to the dorsal arches of the same vertebrae in 

 Scomber. The occipital arch of Necturus would then represent 

 the entire cartilaginous occipitale laterale of Amia, if that 

 structure represents but a single vertebral element, or the 

 posterior one of the three vertebral elements that enter into it, 

 if there are three. Which of these two suppositions, if either, 

 is the correct one can only be known after the investigation of 

 larval stages of Amia earlier than any I have as yet examined. 



Palais Carnoles, 



Menton, April 30, 1898. 



