NOTES ON ELASMOBEANOH DEVELOPMENT. 261 



the limbs. I do not think the fourth slit is open. The posterior end of the 

 mandibular arch is slightly bent ventralwards. 



Pig. 8. — Ventral view of head of same embryo drawn to a slightly smaller 

 scale. The anterior part of the buccal slit has become much wider. 



Fig. 9.— Side view of head of Scyllium canicula about Balfour's 

 Stage K. Total length about 11 — 12 mm. External gills have appeared 

 on the first and second branchial arches. The ventral bend of the hind end 

 of the mandibular arch is more marked. 



Fig. 10. — Ventral view of the same head drawn to a slightly smaller scale. 

 The future angle of the jaw can be distinguished, the moutji being widest at 

 that point. The posterior slit-like part of the mouth is stiU present. 



Fig. 11. — Side view of head of Scyllium canicula about Balfour's Stage 

 L. Total length about 16 mm. The external gills have increased in number, 

 and are present on the mandibular arch. The angle of the jaw where the 

 lower part of the mandibular arch bends ventralwards is very marked. 



Fig. 12. — Ventral view of same head drawn to a smaller scale. The mouth 

 has much widened, and the posterior slit-like part has almost entirely dis- 

 appeared. The anterior part of the mandibular arch has a process towards 

 the middle line. The hinder end of the body has been tilted upwards so as to 

 bring the frouto-nasal process into view. 



