HEAD OF SQUALUS 459 



are shown in Table II. Brans (2), Hoffmann (9), Sewertzoff 

 (17), and Neal (11) adopt this view. 



The difference between the two interpretations is centred 

 in the region of the spiracle and hyoid arch. On the first 

 view all the segmented elements are harmoniously and con- 

 secutively related without gaps or discrepancies : on Van 

 Wijhe's there is one somite too many in the region of the 

 spiracle. For since the mandibular or second somite corre- 

 sponds to the first (mandibular) visceral arch, if the second 

 (hyoid) arch corresponds to the fourth somite, as Van Wijhe 

 supposes, then the third somite has no arch or cleft. Van Wijhe 

 suggests that these have been lost. 



The question has been gone into thoroughly in the case 

 of Scy Ilium canicula by Professor Goodrich (8), and it 

 was at his suggestion that I undertook to investigate S q u a 1 u s 

 acanthias (A cant bias vulgaris) in otder to see whether 

 the conditions were similar in this related form. 



The first part of this paper deals with the question of the 

 correspondence in the region of the hyoid arch, and which 

 somite forms the first permanent myotome. This is followed 

 by a brief description of the occipital region, for the purpose 

 of comparing the extent of the cranial region in Squalus and 

 Scyllium. 



The work was done in the Department of Comparative 

 Anatomy at Oxford. To Professor Goodrich, for advice and 

 encouragement, I wish to offer my grateful thanks. I also 

 had the privilege of consulting Professor Neal in person and 

 to him, for valuable assistance and material, I express my deep 

 gratitude. 



In a 4-5 mm. embryo (Text-fig. 1) reconstructed from 

 longitudinal vertical sections all the somites of the head can be 

 discerned. Two visceral clefts are present — spiracle and gill-slit 1 

 situated beneath the third and fourth somites respectively. 



In the next stage (Text-fig. 2, 5 mm.) the second gill-slit has 

 appeared beneath somite 5. Of the dorsal nerves trigeminal 

 is related to the second somite, facial is situated between 



