78 PROCEEDINGS OF THE NATIONAL MUSEUM vol. lie 



Alopias vulpinus. The species with the lowest precaudal count (44) 

 is Squaliolus laticaudus and the one with the highest (149) is Prionace 

 glauca. The lowest count (13) of caudal vertebrae is for Eupro- 

 tomicrus bispinatus, Squaliolus laticaudus, and Squaliolus sarmenti, 

 and the highest 0298) is for Alopias vulpinus. 



Compared with the other suborders, the Squaloidea have low 

 vertebral counts. In all recent systems of classification the Squa- 

 loidea are regarded as relatively advanced, so there is some justifica- 

 tion for regarding theii- low counts as an advanced feature. On this 

 basis, within the Squaloidea we could consider the generally lower 

 counts of the dalatiids as indicating that they are more advanced 

 than the squalids. The Notidanoidea and the Heterodontoidea, 

 which, on other features, have some claim to being representative of 

 the ancestral or primitive sharks, do not have more vertebrae than 

 do many of the so-called advanced sharks in the Galeoidea. This 

 may merely indicate that median numbers are most primitive, with 

 both decrease and increase as specialized conditions. 



Only one family, the Alopiidae, with one genus, Alopias, is recog- 

 nizable on its vertebral numbers alone. The distinctiveness of 

 Alopias is due to the high number of vertebrae in its tail, a feature 

 that might be expected from the extreme length of its tail compared 

 to other sharks. The diagnostic value of vertebral numbers increases 

 in the lower taxa. Thus, in the families containing several genera, 

 a few genera {Lamna in the Lamnidae, Brachaelurus and Stegostoma 

 in the Orectolobidae, Conoporoderma and Atelomycterus in the Scyli- 

 orhinidae, Prionace in the Carcharhinidae, and perhaps others) are 

 clearly recognizable on precaudal, caudal, or total count. Of the 

 genera containing two or more species, almost half include at least 

 one species distinguishable on vertebral counts. Because of our 

 incomplete coverage, we believe it premature to list these species here. 



The suborders Squaloidea, Pristiophoroidea, Squatinoidea, and 

 Heterodontoidea are consistent in having fewer caudal than precaudal 

 vertebrae. In the suborder Galeoidea, the families Scyliorhinidae, 

 Triakidae, and most of the Orectolobidae show the same trend. It 

 may be of significance that the sharks with fewer precaudal than 

 caudal vertebrae are for the most part small, and usually bottom- 

 dwelling. In Carcharhinus (Garrick, in ms.), the large species tend 

 to have fewer caudal than precaudal vertebrae and the small species 

 have more caudal than precaudal vertebrae. 



In our studies, particularly of the genera Carcharhinus, Scoliodon, 

 Loxodon, and Bhizoprionodon (Garrick, in ms.; Springer, 1964), we 

 have noted no sexual dimorphism in vertebral numbers; however, 

 Punnett (1904), who dissected 567 specimens of Etmopterus spinax 



