MUSEUM OF COMPARATIVE ZOOLOGY. 219 



the choroid gland comes from the hyoidean demibranch (Nebenkieme). 

 The gland usually lies within the bony orbit, and with very few excep- 

 tions it is present in those species possessing a pseudobranch (i. e. in this 

 case of course the hyoidean demibranch or its rudiment). The man- 

 dibular pseudobranch of Elasmobranchs and Ganoids lies behind the 

 orbital territory, but there are cases in which an evident approach to 

 the orbit is recognizable. The vessels of the mandibular pseudobranch 

 consist of an afferent and an efferent artery, as in the perfect branchiae, 

 but usually they are shifted in position, so as to run more or less par- 

 allel to the long axis of the body, instead of transverse, as in the normal 

 condition. The afferent trunk leaves the hyoidean efferent branchial 

 just before the latter leaves the arch and passes forwards to end in 

 the rete mirabile of the spiracular gill 3 while the efferent trunk arising 

 from the rete passes forwards and inwards across the hind portion of the 

 orbit into the cranial cavity, where it unites with the dorsal aorta by an 

 anastomosis with the internal carotid, near the origin of the ophthalmic 

 artery. The homology of the mandibular artery of Callorhynchus, as 

 given by Parker, involves a mistaken identity, as we readily perceive by 

 referring to the author's own works on the Skate and Mustelus antarcti- 

 cus, as well as by reference to figures by Hyrtl (3) and Midler (6). It 

 seems to me clear that the vessel designated posterior carotid by Parker 

 is the arteria vertebral is. 



It is of course possible that the arteriae vertebrales of the Skate are a 

 pair' of musculo-spinal branches of a now vanished dorsal aorta, but from 

 their prominent connection with the first efferent branchials of the Skate 

 it is more probable that they are reduced efferent branchials — of the 

 mandibular gill? The relations of the afferent and efferent vessels to 

 the spiracular gill in Chlamydoselachus — a few leaflets of which still 

 persist — I have not worked out satisfactorily as yet. 



There are traces of other lateral branches to be found in the cartilage 

 at either side of the aorta, between the occiput and pituitary promi- 

 nence. In two sections I saw lateral unpaired vessels passing out from 

 the median line to fade out in the cartilaginous tissue which appeared 

 to be the cause of their suppression. Heptabranchias shows similar 

 vessels. (See Figure 9.) They are so short and indistinct that it is 

 with difficulty they can be traced without entirely destroying the carti- 

 laginous floor of the skull in shaving it down thin enough to see them. 

 The microscopic sections prepared from one of these transsections 

 showed onry a fibrous cord entirely destitute of a cavity. Presumably 

 then the vessels were functional only during embryonic life. There is 



