PERRIN ET AL.: GROWTH AND REPRODUCTION OF THE EASTERN SPINNER DOLPHIN 



FIGURE 3.— Length-frequency dis- 

 tributions, by 5-cm increments, of col- 

 lected female eastern spinner dolphins 

 by year and month. Shaded squares are 

 fetuses. Sample sizes in parentheses. 



(Felker 6 model 80BQ Hi-speed Precision Cut-off 

 Machine — 36,000 rpm) with a single blade, as for 

 S. attenuata (Perrin et al. 1976), but most were 

 sectioned with a single cut of tandem blades (yield- 

 ing a section of uniform 10/1,000-in thickness) 

 with a low speed saw (Isomet model 11-1180 low 

 speed saw — speed variable to 300 rpm). The latter 

 method yielded sections of more uniform thickness 

 and with fewer extraneous surface striations than 

 did the former. After cutting, sections were im- 



6 Reference to trade names does not imply endorsement of the 

 product by the National Marine Fisheries Service, NOAA. 



mediately mounted under coverslips on micro- 

 scope slides in balsam, omitting the clearing step 

 used for the S. attenuata teeth. Our reading and 

 scoring methods also differed from those in the 

 earlier study. We found postnatal dentinal growth 

 to be much more consistent in S. longirostris than 

 in S. attenuata in pattern of deposition and in 

 readability. No teeth were encountered that did 

 not contain an open cavity, albeit in older speci- 

 mens a very small one, and a smaller proportion of 

 teeth from younger animals (<12 to 13 layers) 

 contained convoluted secondary dentine than in S. 

 attenuata. In the study of S. attenuata, growth 



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