The Anatomy of Chlamydoselachus 399 



Heptanchus (Vetter, 1874, Fig. 1, pi. XIV) the muscle is well developed; in Chlamy- 

 iosdachus it is apparently represented by a ligament which is attached, not to the mandi' 

 ble, but to the hyomandibular. It seems probable that the presence of this muscle, as 

 in Heptanchus, is primitive for sharks while the related structure in Chlamydoselachus 

 is a modification that has arisen in connection with the peculiar hyostylism of the jaws. 



In attempting to identify homologous muscles in different species of vertebrates, 

 considerable dependence is placed on their innervation. The motor nerves, growing 

 outward from the central nervous system, establish connections with the muscles or 

 pre'muscle masses quite early in their development. Should the muscle subsequently 

 migrate in order to reach its definitive position, its nerve follows it. Thus in the branchial 

 region, it is generally considered that all the muscles innervated by the fifth (trigeminal) 

 nerve are derivatives of the first visceral (the mandibular) arch, while all the muscles 

 innervated by the seventh (facial) nerve are derivatives of the second visceral (the hyoid) 

 arch. In most sharks, the musculus intermandibularis is supplied by the mandibular 

 branch of the trigeminal nerve; but in Chlamydoselachus, Furbringer (1903, Fig. 1, Taf. 

 XVI) figures the musculus intermandibularis as supplied only by branches of the seventh 

 (the facial) nerve, and Hawkes (1906) states that "the mandibular ramus [of the trigeminal 

 nerve] does not supply the large median muscles which lie in the angle made by the two 

 sides of the lower jaw." Luther (1909) was unable to trace any branches of the trigeminal 

 nerve to the intermandibular muscles of Chlamydoselachus, Hexanchus and Heptanchus. 

 In the notidanids and in Chlamydoselachus, the superficial muscles spanning the halves 

 of the mandible are supplied by branches of the seventh or facial nerve (Luther, 1909). 

 For Chlamydoselachus and Heptanchus the distribution of these branches is shown in 

 Luther's (1909) Fig. 1, Taf. I, and Text'figs. 9 and 10; for Chlamydoselachus they are 

 better shown by AUis (1923) in his Fig. 6, pi. VI, which is in color. Luther (1909) 

 concluded that when the intermandibular muscle is innervated wholly by the nervus 

 facialis, a muscle of mandibular^arch origin has simply been crowded out by one of hyoid 

 arch origin; but in his later work (1913, p. 46) Luther decided that the trigeminus muscle 

 here persisted, but had secondarily acquired innervation by the nervus facialis. 



AUis (1917) gave particular attention to this matter of the innervation of the 

 musculus intermandibularis in Chlamydoselachus and related forms. His conclusions 

 appear to be embodied in the following statement (AUis, 1917, p. 389) : 



The interhyoideus and intermandibularis muscles of Chlamydoselachus could accordingly 

 both be of facialis origin, so far as the relations of nerve and muscle are concerned, but in all 

 probability only that portion of the intermandibularis that Hes anterior to the point where 

 the nervus faciahs definitely disappears from its external surface could be of mandibular 

 origin. And if this portion of the muscle be of mandibular origin, as several authors have 

 maintained, I consider it certain that it is innervated by a branch of the nervus mandibularis 

 trigemini, and that that branch has simply been missed in dissections, my own included. 



In the introduction to his 1923 memoir, AUis states: "The investigation of the 

 nervous system had only just begun, and . . , this part of the cranial anatomy is only 



