14 



PROCEEDINGS OF THE NATIONAL MUSEUM 



pores on the ventral surface of the head, appear to change very Httle 

 throughout life, whereas the shape of the fins may be modified with 

 an increase in body size. Other postcephalic features seem to remain 

 fairly constant. 



Negative allometric growth occurs in the head of sphyrnids since, 

 with an mcrease in total body length, there is a proportional decrease 

 in the following characters: Width of head, tip of snout to nares, tip 

 of snout to orbit, tip of snout to symphysis of jaws, diameter of orbit, 

 and intranarial width. 



The shape of the upper precaudal pit, changing with an increase in 

 size of the individual, undergoes the greatest modification during 



HYPOTHETICAL PHYLOGENY 

 OF THE GENUS SPHYRNA 



GARGHARHINID ANCESTOR 



Figure 4. — Hypothetical phylogeny of the genus Sphyrna (figure by Paul Laessle). (S. 

 couardi, which was not included, is closely related to S. lezvini.) 



early growth and likely reaches its final form before the shark is 1,000 

 mm. long; this is illustrated, from specimens of Sjjhyrna media 

 measuring 356 and 900 mm. TL, in figures 21/, g. 



Other morphometric changes are indicated with an increase in size 

 (tables 2-9) , although more large specimens must be examined before 

 the validity of these changes can be established. Pronounced modifi- 

 cations, particularly in length of the trunk between the axil of the 

 pectoral fin and the insertion of the pelvic fin, have been shown to 

 occur in squaloid sharks (Garrick, 1960, pp. 546-548). Since most 

 squaloids do not reach a very large size, however, considerable changes 

 in body proportions can occur with a relatively small increase in total 



