370 Bashford Dean Memorial Volume 



Concerning the occurrence of cyclospondylous centra, Goodey (1910.1) writes 

 as follows: 



The points at which the calcified centra occur are perhaps deserving of some mention. 

 It seems that they are found where there are the greatest demands made for strength. At 

 the anterior end, combined with the fusion of the vertebral column to the cranium, they 

 give a rigidity to the supporting elements which is of service no doubt in enabUng the fish to 

 cleave the water. In the caudal region they meet the demand for increased strength caused 

 by the purchase which the caudal fin obtains upon the water. 



It might be added that in the caudal region the cartilaginous bridges across the 

 lateral surfaces of the chordal sheath give greater strength to the vertebral column. On 

 the other hand, the diplospondylous condition gives greater flexibility (Ride wood, 1899). 

 In general, the vertebrae are best developed in the region that is subjected to the most 

 severe stresses. 



We have seen that the vertebral column of Chlamydoselachus is of interest in 

 a number of ways. The notochord persists, in the adult, with so little modification 

 that it is one of the most primitive known in living sharks. The cartilaginous elements 

 of the vertebral column are of a very simple elasmobranch type and illustrate various 

 stages in the formation of complete vertebrae. In the cervical and caudal regions one 

 finds early stages in the formation of cyclospondylous centra; these arise as calcifications 

 in the chordal sheath. In the main region of the tail the dorsal and ventral arcualia are 

 connected by cartilaginous bridges, giving unity and completeness to the structure of 

 each vertebra. In the region of transition from body to tail, monospondylous vertebrae 

 gradually give way to diplospondylous vertebrae. Finally, at the extreme tip of the tail 

 there is a condition of heterospondyly which is perhaps unique among selachians. 



APPENDICULAR SKELETON 



The appendicular skeleton of Chlamydoselachus includes the cartilaginous frame' 

 work of the pectoral and pelvic fins, together with the pectoral and pelvic girdles; and 

 the cartilaginous supports of the dorsal and anal fins. The endoskeletal supports of the 

 tail fin belong mainly to the axial skeleton, but it is convenient to consider the framework 

 of the caudal fin along with the skeletons of the other fins. 



PECTORAL FINS AND GIRDLE 



The skeleton of the pectoral fin of Chlamydoselachus has been described and figured 

 by Carman (1885.2); Braus (1902); Deinega (1909 and 1923); and Goodey (1910.1). The 

 pectoral girdle or coraco-scapular (Text-figures 37 and 38) bears a decided resemblance 

 to that of Heptanchus (Daniel, 1934, Fig. 54) ; but in the fin proper the radials of Chlamy^ 

 doselachus are relatively shorter, and are segmented to form typically three rows of 

 cartilaginous elements while Heptanchus has about twice that number. 



Braus's figure of the pectoral fin skeleton of Chlamydoselachus portrays a ventral 

 view. It differs from Carman's figure (aspect not stated) in a number of details, as may 



