Pseudobranchial and Carotid Arteries in the Gnathostome Fishes. 107 



accordingly placed in my diagram in the position of such a gill; 

 and 'that the gland, being a premandibular gill, and having lost 

 its primary connection with the ventral aorta, would be supplied 

 by a commissural vessel from the next posterior arch, rather than 

 by the dorsal remnant of its own aortic arch, seems wholly 

 probable. 



The external carotid is shown arising from that section of the 

 dorsal aorta that lies between the dorsal ends of the hyoidean and 

 mandibular arches, but this origin of the artery is apparently not 

 constant in selachians. Paekee (1886) gives it this origin in 

 Mustelus antarcticus, Hyetl (1872) in AcantJiias vulgaris and Zijgaena 

 malleus^ and Caeazzi (1905) in Scyllium catuliis and Selache maxima; 

 but Hyetl shows it arising from the hyoideo-glossopharyngeal section 

 of the aorta in Scyllium canicula, and Caeazzi and Ayees (1889) 

 show it arising from the dorsal portion of the eiferent hyoidean 

 artery in Squatina vulgaris and ChlaniydoselacJms anguineus, respectively. 

 And if this latter point of origin, from the dorsal portion of the 

 hyoidean aortic arch, is the primitive arrangement, it might be taken 

 to indicate that the artery results from the unusual development of 

 what was primarily either a dorsal commissure between the hyoidean 

 and mandibular aortic arches alone, or a series of such commissures 

 between a number of prehyoidean arches. Ayees (1889) ascribes 

 this latter origin to the continuous vessel formed by the common 

 and internal carotids, but that can evidently not be so if those tw^o 

 arteries are parts of the lateral dorsal aorta. And even while it 

 may be true of the external carotid, it seems more probable that 

 that artery is developed from a dorsal branch of the hyoidean aortic 

 arch, similar to those dorsal branches that actually arise from 

 certain of the branchial arches and go to supply the dorsal muscles 

 of the related region. The arteria temporalis, or temporo-maxillaris 

 of Hyetl's (1858j descriptions of the Batoidei is such a nutritive 

 or muscle arterj'^ related probably to the first branchial arch; as 

 are also the opercular and dorsal branchial muscle arteries of 

 Allen's (1905) descriptions of the Loricati, the branches there 

 being related to the second branchial arch. Dohen suggests 

 (1890, p. 393) that the external carotid may represent a remnant 

 of the aortic vessel of some visceral arch, and it is evident that 

 certain of the branches of the artery have a distribution that 

 suggests their representing, and perhaps being derived from, the 

 ventral portion of a premandibular aortic arch. Such an origin 



