The Anatomy of Chlamydoselachus 



379 



Osburn has published a drawing (1907, Fig. 19, pi. V) of the dorsal fin skeleton of 

 a 225'mm. embryo of Chlamydoselachus. The total number of cartilaginous elements 

 (thirty-'six) is smaller than in Carman's specimen (fortyfive), and much smaller than in 

 Deinega's specimen (sixty'one). The larger number in the adult may possibly be due to 

 fragmentation. Osburn notes the wide separation of the dorsal fin skeleton from the 

 axial skeleton. 



In the absence of any further examples it appears that the entire endoskeleton of the 

 dorsal fin of Chlamydoselachus is composed of radials. Some segments of these radials 

 have undergone slight displacement, but there is Httle or no fusion. In Heptanchus 

 cinereus (Text'figure 50) the radials (ra.) of the dorsal fin are much more regular and there 



Text-figure 50. Text-figure 51. 



Endoskeletons of the dorsal fins of Heptanchus and Mustdus. 



Text-figure 50. Cartilages of the dorsal fin of Heptanchus cinereus. 



be, basal; ra., radial cartilage. 

 From Daniel, 1934, Fig. 56; after Mivart, 1879, Fig. 2, pi. LXXV. 



Text-figure 51. Cartilaginous elements of dorsal fin of Mustelus antarcticus. 



b.c, basal segments; h.c.l, median segments; b.c.2, distal segments. 

 From Daniel, 1934, Fig. 89a, after Mivart. 



is a large but thin basal cartilage (be). In Mustelus (Text-figure 51) there is a distinct 

 row of basal cartilages (b.c.) that appear to have been segmented off from the radials, 

 but there is no fusion. 



There is no need of recourse to fossil forms to find evidence of the manner of origin 

 of basal plates in the dorsal fin skeleton. Beginning with the condition exemplified by 

 Mustelus, which I regard as primitive, there may be found in living forms all intermediate 

 conditions leading to one in which fusion of basal segments of the radials has formed 

 large basal plates. The literature pertaining to the fin skeletons of sharks abounds in 

 figures which, upon comparison, illustrate the point, but it is sufficient to cite Mivart's 

 (1879) well-known drawings. In Chlamydoselachus the endoskeleton of the dorsal fin, 

 though primitive, seems to have suffered regression as evidenced by the irregular form 

 and arrangement of many of the cartilaginous elements. 



