The Anatomy of Chlamydoselachus 



485 



dency toward unification of the system. The trigemino'facial complex is less primitive 

 than that of Chimaera, but more so than that of most elasmobranchs. Hawkes' general 

 conclusion is that the cranial nerves of Chlamydoselachus are not in so primitive a con' 

 dition as would be expected from the low position of the species in the taxonomic series, 

 especially as regards the vagus and the lateralis nerves. 



THE SPINAL NERVES 



Hawkes' description (1906, pp. 985-987) of the spinal nerves of Chlamydoselachus 

 is concerned mainly with the spinal nerve roots. I quote her account almost entire: 



The ventral root of the first true or complete spinal nerve originates between the first 

 and second vertebrae. Spinal nerves 1, 2, 3, 4, 5 (Fig. 1, pi. LXVIII) [my Figure 29, plate 

 VII] unite with the spino'occipital nerves into a strand, which passes backwards, then out' 



Text-figure 121. 

 Diagram of spinal nerves from anterior, middle 



and tail regions of Chlamydoselachus. 



C.S., connecting strands between dorsal and ventral roots; 



D.B., dorsal branch; D.R.G., dorsal root with its ganglion; 



'Ho., notochord; S.7^., spinal nerve; V.B., ventral branch; 



V. C, vertebral column; V. R., ventral root. 



After Hawkes, 1906, Text-fig. 141. 





wards towards the pectoral girdle. Spinal nerves 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.l and S.h.2). Branch S.h.l, which is connected 

 with vagus 6, passes forwards and downwards to join branch S.h.2. The resulting compound 

 nerve passes forward near the median ventral line to supply a portion of the median man- 

 dibular or hypoglossal musculature. It is probable that this nerve consists only of fibers 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 composite strand, i.e., the 

 first eight complete spinal nerves of which the last remains distinct. The brachial plexus is 

 here in a simple condition, for it consists of but few nerves, and those are not intimately 

 united. . . . 



Each spinal nerve arises by two alternate roots, a dorsal and a ventral. The ventral root 

 [V.R.] arises by three rootlets, then, after emerging from the vertebral column, gives off 

 a large dorsal branch (Text-fig. 141, D.B.) [my Text-figure 121] before uniting with the dorsal, 

 ganglionated root [D.R.G.]. In the anterior and middle regions of the vertebral column, 

 this union takes place at a level with the top of the notochord, but in the tail region at a level 

 with the base of the notochord, immediately to the inner side of the ramus lateralis vagi. 

 The ventral branch (V.B.) is given off at varying points (Text-fig. 141) [my Text-figure 121]. 



The dorsal branch (D.B.) of the ventral root runs caudad and upwards, passing over 

 the ganghon of the dorsal root (D.R.G.) to be distributed to the muscles of the middle region of 

 the back. A similar root (ventral-dorsal) has been described by Ewart and Cole in Raia. No 

 dorsal branch was found for the complete spinal nerve or for the dorsal root, as it is probable 

 that the dorsal branch of the ventral root receives fibres from the dorsal root as it passes 

 over the latter on its backward course. In one segment (Text-fig. 141) [my Text-figure 121] 



