The Embryology of Heterodontus japonicus 



763 



that most of the observed differences are correlated with age and use. The most important 

 points may be summari2ied as follows: The most anterior of the cuspidate teeth begin, as 

 a rule, with five cusps, but some of the more posterior cuspidate teeth begin with only 

 three or four cusps. The typically five'cusped condition of the anterior teeth persists 

 until long after hatching. Before the adult (sexually mature) stage is reached, the number 

 of cusps in these teeth is reduced to three, with the central cusp most prominent. Gradu' 

 ally the lateral cusps become inconspicuous or even absent. With age and use (the food 

 consisting mainly of molluscs, crustaceans, and sea urchins) the central cusp may become 

 worn down until the anterior teeth, collectively, appear almost pavement-like. (The 

 word pavement, as used here, refers to the old-fashioned much-worn stone-block pave- 

 ment). The posterior or grinding teeth never have prominent cusps, and some are 

 entirely without cusps. The few rudimentary cusps that appear in the early stages soon 

 give place to a longitudinal ridge, useful in grinding the food. In older specimens, this 

 ridge may be entirely worn away, leaving the tooth with a smooth rounded surface. 

 Thus the posterior teeth become more pavement-like than the anterior teeth; in the adult 

 they are much larger and stronger. In their prime, the anterior teeth are well-fitted for 

 prehension, the posterior teeth for crushing and grinding. All the descriptions and 

 illustrations of both young and adult teeth emphasise the differences between anterior 

 and posterior teeth — differences that suggested the generic name, Heterodontus. 



Another view (see also Text-figure 1) of the Marine Zoological Station at Misaki where 

 Dr. Dean studied the Japanese Bullhead Shark. 



