The Embryology of Heterodontus japonicus 



697 



Text-figure 29. 

 Teeth of Hybodus, outer aspect, natu- 

 ral size: A, three associated teeth of 

 Hybodus delahechei Charlesworth ; B, 

 three associated anterior teeth of 

 Hybodus reticulatus Agassiz. 

 After Woodward, 1889, Part 1, pi. X. 



in some larger group; but we are here concerned mainly with the interrelations of the 

 two families. From this point of view, the descriptions of the teeth by Woodward are 

 inadequate when isolated from the special accounts of the teeth of the various genera. 

 There is considerable variation in the teeth of different genera in both families, and the 

 differences are of the same kind. 



In Hybodus the teeth (Text-figures 29 and 30) are all cuspidate. In the anterior teeth 

 the cusps are more or less acute, with the central cusp predominant and the other cusps 

 somewhat irregular in sizie and number. In the posterior teeth there is a tendency toward 

 differentiation into grinders; for these teeth are larger than the anterior teeth and their 

 cusps are almost or quite obtuse. But in some other genera of the family Hybodontidae, 

 low rounded crushing teeth, slightly ridged and with only a few vestigial cusps, occur 

 (e.g., as in Orodus, figured by Eastman, 1903; and Acrodus, beautifully illustrated by 

 Woodward, 1889). 



Similar differences occur in the three genera of the Cestraciontidae. The teeth of 

 Synechodus (Text-figure 31) are much like those of Hybodus (Text-figures 29 and 30) 



Text-figure 30. 



Posterior teeth of Hybodus, in natural sizes. A, Hybodus delabechei Charlesworth: four 



posterior series of teeth, coronal aspect; one tooth of each of three series is shown also 



in side view. B, Hybodus raricostatus Agassiz: two posterior series of teeth and portions 



of a third, coronal aspect; two teeth are shown also in side view. 



After Woodward, 1889, part 1, pi. X. 



