88 PLATT. [Vol. V. 



homologues of these anterior cavities in Galeus and Scyllium 

 may be regarded as the visceral portions of the premandibular 

 cavity cut off by the mouth involution. Such a view is not 

 warranted by the position of the cavities in Acanthias. They 

 lie dorsal to the premandibular cavity, and if an independent 

 segmental value is not to be ascribed to them, it is much more 

 probable that they represent the visceral portion of the anterior 

 division of the mandibular cavity than that of the premandibular, 

 as suggested by Van Wijhe (No. 16). 



B. Circulation. 



The early development of the circulatory system in Acanthias 

 corresponds quite closely to the development in Pristiurus as 

 described and figured by Dohrn (No. 6). A few differences, 

 however, may be noticed. 



At the stage from which Fig. 8 is reconstructed, from the 

 enlarged end of the primitive heart, two arteries ascend in 

 the mandibular arches to the dorsal aorta. At the base of the 

 mandible each artery becomes enlarged, and a downward pro- 

 jection of its ventral wall as the artery turns inward to meet 

 that of the opposite side, is the budding arteria ophthalmica 

 magna. Anterior and lateral to the two projections thus 

 formed lie the lateral portions of the premandibular cavity. 

 The ophthalmicae magnas are thus seen to maintain in their 

 position the same relations to the lateral portions of the pre- 

 mandibular cavity that the hyoid and mandibular arteries main- 

 tain to their respective head cavities. Their position is median 

 and slightly posterior to that of the cavities. If the ophthalmica 

 magna is, as Dohrn (No. j) suggests, to be regarded as the 

 remains of a premandibular aortic arch, then it would seem 

 reasonable from the position of the vessel to connect it with 

 the premandibular cavity, as both belonging to the same body 

 segment. 



As the mandibular arteries fuse with one another, they send 

 forward two small branches, which, passing to either side of the 

 notochord, open into a wide blood-space posterior to the point 

 of the chorda. From this space two branches extend laterally 

 downwards and forwards (Fig. 8, ac, PI. IV). These branches 

 lie at either side of the premandibular cavity, but meet the 



