FISHERY BULLETIN: VOL. 75, NO. 4 



layers were merely counted to the nearest half 

 layer. In view of the better readability of the S. 

 longirostris material, we felt that the approach 

 could be refined. We measured the thickness of 

 each growth layer on an image projected on a 

 16-cm diameter ground-glass screen attached to a 

 compound microscope using dial calipers accurate 

 to 0.1 mm. The total magnification on the screen 

 was approximately 263 diameters. The first two 

 layers inside the neonatal tooth were measured at 

 approximately the same point along the length for 

 each tooth, about halfway between the proximal 

 end of the neonatal tooth (point where neonatal 

 line meets outer surface) and the distal end of the 

 neonatal pulp cavity. Layers beyond the second 

 were measured at the place along the length of the 

 tooth where they were most clearly defined. In 

 converting measurements to layer units, non- 

 innermost layers beyond the first layer were 

 scored as full layer units regardless of thickness. 

 The first layer, second (when innermost), and sub- 

 sequent layers (when innermost) were treated dif- 

 ferently, as follows: 



First Layer 



Cumulative percent of 417 first layers measured 

 rapidly approaches an asymptote at approxi- 

 mately 0.20 mm (Figure 4). A first layer 0.20 mm 

 thick or thicker was therefore scored as a full layer 

 unit, and the thickness of a first layer <0.20 mm 

 thick was divided by 0.20 mm to yield a partial 

 layer unit. 



Second Layer 



In 361 teeth with three or more layers, the sec- 

 ond layer averaged 0. 145 mm thick with relatively 

 low variance (Figure 5). In teeth with two layers, 

 the second layer was scored as a full layer unit if 

 0.145 mm thick or thicker. An innermost second 

 layer <0.145 mm thick was scored as a partial 

 layer unit by division of the thickness by 0.145 

 mm. 



Layers Beyond Second Layer 



Full layers beyond the second layer averaged 

 more than 95% of the thickness of the next older, 

 adjacent layer, with considerable variation that 

 increased toward the center of the tooth (Figure 6). 

 We assumed, as a reasonable approximation, that 

 complete layers beyond the third are of about the 



730 



100 r 



— 80 - 



005 



0.10 0.15 020 025 



THICKNESS OF FIRST LAYER (mm) 



030 



FIGURE 4. — Cumulative percent of first growth layers in rela- 

 tion to thickness of layer in the teeth of 417 eastern spinner 

 dolphins showing asymptote of sigmoid curve at about 0.20 mm. 



AVERAGE = 145 mm 



008 012 0.16 020 



THICKNESS OF SECOND LAYER (mm) 



028 



FIGURE 5. — Frequency distribution of thickness of second 

 growth layer in teeth of 361 eastern spinner dolphins. 



same thickness as adjacent layers, and thickness 

 of the innermost layer in teeth with three or more 

 layers was scored as a proportion of the next older, 

 adjacent layer. Layer scores thus obtained were 

 added and rounded off to the nearest tenth of a 

 layer. 



Brain weights were obtained from brains dis- 

 sected out of freshly thawed heads or whole car- 

 casses of specimens deep frozen at sea aboard 

 tunaboats, except for nine weights for S. attenuata 

 (two fetuses, two neonates, and four adults) ob- 

 tained from George A. Sacher (Argonne National 

 Laboratory, Argonne, 111., pers. commun.). 



Testes were weighed with the epididymes at- 

 tached. 



