200 BULLETIN OF THE 



this instance the vessel is atrophied during the process of the reduction 

 of the anterior gill, and the blood which formerly passed through it 

 direct to the collecting vessel above its gill now passes backward, 

 and its segment of the collecting vessel is now either carotid or 

 carotid root. 



Since the number of efferent branchial arteries uniting to form the 

 dorsal aorta varies in different species, the dorsal aorta cannot be an 

 equivalent structure throughout the vertebrate series. 



We know that the cranium itself is not an equivalent structure within 

 the limits of the Elasmobranchii. Abundant proof of this has been 

 collected by Gegenbaur, 12 Froriep, 13 Van Wijhe, 14 Dohrn, 16 and others. 

 Evidence based on the relation of blood-vessels to the cranial floor 

 is worthy of note in this connection. In some Elasmobranchs the an- 

 terior pair of musculo-spinal arteries pierces the cranial floor, in other 

 forms this pair is intimately related to the atlas. Where the former 

 condition is present, we can say definitely that at least one vertebra 

 has been added to the cranium, but in the latter case we may have to 

 deal with a suppression of one or more pairs of musculo-spinal arte- 

 ries, in which case we cannot draw conclusions as to the constitution of 

 the cranium.* 



Since there can be little doubt, if any, that the primordial cranium is 

 the same in all cases, it follows that, during the early stages of on- 

 togeny, segments (cranial vertebrae) must have been added to the occip- 

 ital region in a large number of cases, and we are at once confronted with 

 the difficulty of determining the number of such added segments, and 



12 Gegenbaur, C. Die Metamerie des Kopfes u. die Wirbeltheorie des Kopf- 

 skeletes. Morph. Jahrbuch, XIII., 1887. 



13 Froriep, A. Ueber ein Ganglion d. Hypoglossus u. Wirbelanlagen in d. Oc- 

 cipitalregion. Arch. f. Anat. u. Physiol., 1862. 



14 Van Wijhe, J. W. Ueber die Mesodermsegmente u. d. Entwickl. d. Nerven 

 des Selachierkopfes. Konigl. Acad. d. Wissensch., 1882. 



15 Van Bemmelen, J. F. Ueber vermiithliehe rudimentare Kiemenspalten bei 

 Elasmobranchiern. Mitt. d. zoolog. Stat, zu Neapel, VI., 1885. 



16 Dohrn, Alex. Studien zur Urgeschichte der Wirbelthierkorper. Mitt. d. 

 zoolog. Stat, zu Neapel, VII., 1887. 



* Transections of the basis cranii of Chlamydoselachus taken from the verte- 

 bral junction forward show at intervals calcified tracts leaving the central peri- 

 chordal crust, and extending on either side out into the hyaline cartilage. They 

 correspond in position to the neural arches, transverse processes, and hypapophyses 

 of the vertebrae. (See Figure 8.) It may be possible to determine the number of 

 vertebras entering into the basis cranii of Chlamydoselachus by making a perfect 

 series of such sections and counting the number of these vertebral remains. 



