84 ALUS. [Vol. II. 



cranium in that part of it that lies between the posterior limit 

 of the protometameric cranium of Sagemehl's descriptions 

 (No. 6, p. 526) and the posterior limit of the paleocranium ; 

 and that he considers as spino-occipital nerves all those that 

 issue in that part of the auximetameric skull of Sagemehl's 

 descriptions that lies posterior to its protometameric portion. 



These definitions of these nerves seem at first sight to be 

 morphologically concise and definite. A little consideration 

 will, however, show that two suppositions can be made regard- 

 ing the segmental position of the nerves thus incorporated in 

 the skull. They can either lie, morphologically, between the 

 dorsal arches of two adjacent assimilated vertebrae, and so 

 become enclosed between those vertebrae as they fuse with 

 each other and with the skull ; or they can lie, morphologi- 

 cally, posterior to the dorsal arches of the assimilated ver- 

 tebrae, become first incorporated in those vertebrae and 

 then with them in the skull. Under the first supposition 

 the last nerve incorporated in the skull would lie anterior to 

 the dorsal arch of the last incorporated vertebra, and anterior 

 to the intermuscular septum related to that vertebra. Under 

 the second supposition it would lie posterior to the same arch 

 and septum. Each nerve of the series would accordingly 

 belong, under the first supposition, to a trunk muscle-segment 

 one anterior to the one it would belong to under the second 

 supposition. From Flirbringer's descriptions it is not evident, 

 in each particular case, to which of these two categories the 

 nerves under consideration belong. That, in ganoids and tele- 

 osts at least, he considers them as belonging definitely to the 

 first category is evident from his statement regarding Polyodon. 

 Of this fish he says (No. 2, p. 449) : " Fiir mich bildete das 

 Verhalten an dem untersuchten Exemplare von Polyodon, wo 

 die erste dorsale Wurzel (a) eine durch ein partielles Ligament 

 markirte Stelle des- Schadels passirt, das entscheidende Krite- 

 rium. In dieser Stelle erblickte ich die noch nicht vollkom- 

 men verwischte Grenze zwischen dem selachierartigen (proto- 

 metameren) Cranium und der Wirbelsaule, und in der dorsalen 

 Wurzel diejenige des urspriinglichen ersten Spinalnerven, der 

 nun zum ersten occipito-spinalen Nerven (a) geworden ist." 



