400 Bashford Dean M.emorial Volume 



briefly noticed in the present memoir." This leaves us in doubt whether Allis made any 

 further dissections before writing (1923, pp. 188-189): 



The muscles innervated by the nervus faciaHs, all of which are here considered as belong- 

 ing to the hyal arch, are represented by a single continuous muscle sheet, which is partially 

 differentiated, by differences in the insertion of its fibers, into a constrictor superficialis, 

 a levator hyomandibularis, an interhyoideus and an intermandibularis. . . . These several 

 portions of the continuous muscle sheet are all apparently innervated exclusively by branches 

 of the nervus facialis, and there is accordingly no musculus intermandibularis of mandibular 

 origin in this fish. This has been fully discussed in an earHer work (AlHs, 1917), the course 

 of the ramus hyoideus facialis and its relations to the several muscles there also being given. 



Whatever light future investigations may throw on the possible persistence of 

 a vestigial musculus intermandibularis of mandibular arch origin, the fact remains that 

 what appears to be the intermandibular muscle of Chlamydoselachus, Hexanchus and 

 Heptanchus is innervated by a branch of the facial nerve, contrary to what has been 

 found in all other sharks that have been investigated. This evidence, so far as it goes, 

 tends to draw Chlamydoselachus and the notidanids closer together and at the same time 

 to separate them further from other existing sharks. 



Considering the small size of the external opening of the spiracle and the absence of 

 an authentic spiracular cartilage, it is not surprising that we have found no mention of 

 a special spiracular muscle in Chlamydoselachus. Luther (1909, p. 12) mentions a spirac- 

 ular muscle in Hexanchus, and in his Fig. 1, Taf. I, it is clearly shown as a prominent 

 sphincter; but there appears to be no special differentiation of the muscles adjoining the 

 spiracle of Heptanchus (Luther, 1909, Fig. 2, Taf. I). 



An interesting though probably anomalous condition of the musculi interarcuales 

 dorsales was found by Allis (1915 and 1923) in one of three specimens of Chlamydoselachus 

 studied by him. In the specimen under consideration, the musculi interarcuales dorsales 

 form an almost continuous sheet of muscular and ligamentous tissue in the roof of the 

 pharynx (Text-figure 71)- These muscles are better shown in AlHs's (1923) Fig. 56, pi. 

 XXI, which is drawn from the same specimen but to a larger scale and in color. In the 

 two other specimens of Chlamydoselachus studied by Allis, the individual muscles of 

 the interarcuales dorsales group are better differentiated and there is no common sheet 

 of muscular tissue mesial to the pharyngobranchials. Nevertheless, the related ligamen- 

 tous sheet existed in the two specimens as in the other one, and "extended the full length 

 of the branchial region" (Allis, 1915). From one of the two specimens thus described, 

 Allis's (1923) Fig. 53, pi. XX, was drawn. The condition shown here is more like what 

 is found in Heptanchus (Furbringer, 1897, Fig. 1, Taf. V; Davidson, 1918, Fig. 3), where 

 the muscle is broken up into segments between the respective pharyngobranchial 

 cartilages. Thus we find, in the musculi interarcuales dorsales of Chlamydoselachus, 

 one more example of decided variability. 



