986 



MRS. O. A. MERRITT JIAWKES OX THE 



[Dec. 11, 



branch, which, like the dorsal branches of the succeeding spinal 

 uei'ves, passes upwards and backwards. No dorsal branches were 

 found on the first two spino-occipital roots. In Lcemaryus there 

 are three spino-occijDital nerves, of which the first two had no 

 dorsal roots (6. p. 480). Immediately outside the cranium the 

 nerves unite into a flattened sti'and, the hyj)ogiossal nei've. 



The ventral root of the first true or complete spinal nerve 

 originates between the first and second vertebrje. Spinal nei'ves 

 1, 2, 3, 4, 5 (PI. LXYIII. fig. 1) unite with the spino-occipital 

 nerves into a strand, which passes backwards, then outwards 

 towards the pectoral girdle. Spinal nei'ves 6 and 7 unite with 

 one another before joining this plexus. Spinal nerve 8 runs by 

 its side, but does not actually join. The spinal plexus gives off 

 anteriorly two branches (s.h. 1 and s.h. 2). Branch s.h. 1, which 

 is connected with Vagus 6, passes forwards and downwards to join 

 branch s.h. 2. The i-esulting compound nerve passes forward 

 near the median ventral line to supply a portion of the median 

 mandibular or hypoglossal musculature. It is probable that this 

 nerve consists only of fibres from the spino-occipital nerves, and 

 would therefore be the homologue of the hypoglossal nerve of 

 higher forms. 



The brachial plexus consists of the remaining parts of the com- 

 posite strand, i.e., the first eight complete spinal nerves, of which 

 the last remains distinct. The bi'achial plexus is here in a simple 

 condition, for it consists of but few nerves, and those are not 

 intimately united. The innervation indicates that the pectoral 

 fins of Ghlamydoselachus are made uj) of a smaller number of 

 segments than in many species ; a fact which, according to 

 Goodrich (11), indicates that the fin here is specialised rather than 

 primitive, for Goodiich states that potentially a fin might extend 

 all along the body, and that it is only in the specialised forms that 

 it is restricted to a few segments, which may not even be homo- 

 logous in allied species. 



Text-fiff. 141. 



Spinal nerves from anterior, ruiddle, and tail regions of Clilamydoselaclms. 



C.S. Connecting-strands between 



dorsal and ventral roots. 

 D.B. Dorsal branch. 

 D.R.G. Dorsal root with its ganglion. 

 No. Notochord. 



S.N. Spinal nerve. 

 V.B. Ventral branch. 

 V.C. Vertebral column. 

 V.E. Ventral root. 



Each spinal nerve arises by two alternate roots, a dorsal and a 

 vential. The ventral root arises by three rootlets, then, after 

 emei'ging from the vertebral column, gives ofl' a large dorsal 



