Pseudobranchial and Carotid Arteries in the Gnathostome Fishes. m 



arteiy ' instead of from the dorsal aorta between the hyoidean and 

 mandibular aortic arches; an origin ascribed also to the same artery 

 in Squatina, by Caeazzi (1905). 



A further point in which Chlamydoselachus differs from the 

 other selachians considered, and also from all other elasmobranchs 

 that I find described, is that the point where the dorsal aorta 

 (internal carotid) anastomoses (crosses, Aters), in the median line, 

 with its fellow of the opposite side, lies anterior to the point 

 where the vessel is joined by the mandibular aortic arch instead 

 of posterior to that point. Furthermore, according to Ayers' de- 

 scriptions, the internal carotids do not enter the cranial cavity 

 and there terminate in cerebral branches; for on p. 199 he 

 says that these arteries, after entering tlie pituitary space, unite 

 with the dorsal aorta, and on p. 198 he says that one of several 

 well defined layers of connective tissue "bridges over the pituitary 

 depression, and thus excludes the internal carotids from the cerebral 

 cavity". This statement, and the figure given by Ayers in illus- 

 tration, definitely establish the existence in Clilamydoselachus of an 

 intramural pituitary space immediately in front of and partly beneath 

 the so-called pituitary prominence, which pi'ominence is evidently 

 the Sattellehne of German descriptions of elasmobi-anchs, and the 

 prootic bridge of English descriptions of teleosts: and as the internal 

 carotids of teleosts anastomose while traversing the myodome of the 

 fish, it is evident that the intramural pituitary space of Chlamydose- 

 lachus is a rudimentary myodome not yet opened into the orbit. 

 That such a space must exist in elasmobranchs, and that it represents 

 a rudimentary condition of the myodome, I have fully set forth in 

 a work now in press, but I did not notice, at the time, this de- 

 scription of the space in Chlamydoselachus. It is evident that the 

 internal carotids of Chlamydoselachus must, at some point within 

 this myodomic space, turn upward and enter the cranial cavity as 

 the cerebral branches of those aiteries; these terminal portions of 

 the arteries having apparently been wholly overlooked by Ayers. 



Ayers shows and describes, in Chlamydoselachus, a small median 

 vessel, which runs directly forward from the point where, according 

 to his nomenclature, the dorsal aorta is joined by the third pair of 

 aortic roots; that is, in the nomenclature employed by me, from the 

 point where the lateral dorsal aortae unite to form a single median 

 trunk. This vessel is said by Ayers to extend forward to the 

 pituitary body, and it is called by him the cranial aorta, that being 



