The Anatomy of Chlamydoselachus 363 



there is a small nodule of cartilage, a similar nodule being "found on the opposite side of the 

 head wedged in between the basal one of the chain of three small nodules and the related 

 ceratobranchial. These two Httle nodules are, in position and appearance, strict serial 

 homologues of the two nodules that represent the proximal ends of the sixth hypobranchials, 

 and they are accordingly quite probably the corresponding ends of the seventh hypobranchials, 

 the posterior process of the large cardiobranchial then being the seventh basibranchial. 



A comparison of all the available figures of the seventh branchial arch in Chlamy- 

 doselachus shows that this arch is extremely variable and is never fully developed. I am 

 inclined to think that phylogenetically it is in process of disappearance rather than in 

 process of development. A rudimentary ninth branchial arch is present in Heptanchus 

 (Daniel, 1934, Fig. 50b). 



It is in the ventral portion of the branchial skeleton of selachians that the greatest 

 amount of variation takes place. A complete series of basibranchials and hypobranchials, 

 without fusion, is presumably the primitive condition, but so far as I know this condition 

 is not fully realized in any living fish. Chlamydoselachus and the notidanids probably 

 come the nearest. Gegenbaur's drawing (1872, Fig. 1, pi. XVIII) of the branchial skeleton 

 of Heptayichus is here reproduced as Text'figure 27. The first basibranchial is lacking 

 and the sixth and seventh are fused together. If one compares Fiirbringer's drawing of 

 Heptanchus (1903, Fig. 29, Taf. XVIII), and DaniePs illustration (1934, Fig. 50a) re- 

 produced as my Text'figure 28, one finds in the basibranchials of Heptanchus quite as 

 much irregularity as I have noted for the same structures in Chlamydoselachus. On the 

 other hand, in Heptanchus the hypobranchials form a more nearly perfect series, especially 

 if one considers the vestigial first and seventh pairs figured by Daniel (my Text-figure 28). 

 In Hexanchus (Gegenbaur, 1872, Fig. 2, Taf. XVIII; FGrbringer, 1903, Fig. 19, Taf. 

 XVII), the basibranchials resemble those of Chlamydoselachus as figured by Goodey 

 (my Text-figure 26a). In respect to both basibranchials and hypobranchials, Chlamy-- 

 doselachus and the notidanids are primitive, yet so variable that they seem to possess the 

 materials for a rapid evolutionary change. 



Text-figure 29. 



Longitudinal section of vertebral column and notochord in the cervical 



region of Chlamydoselachus. 



ch, notochord; in, interdorsal; is, interspinous process; tic, neural canal. 

 After Garman, 1885.2, Fig. 3, pi. X. 



