576 EDWARD PHELPS ALLIS, JR.. 
tion of certain of those rays to the purpose of keeping the bran- 
chial clefts open and so preventing a possible stoppage of the flow 
of water through them (Firbringer ’03, p. 435). According to 
Gegenbaur (’72, pp. 164-166) they are archaic and very variable 
structures, inherited from the common ancestor of the Selachi 
and Cyclostomata and now in process of reduction and disappear- 
ance, and, as rudiments, only, of the dorsal ones were frequently 
found by him when the ventral ones were well developed, Gegen- 
baur concluded that the dorsal ones were subject to reduction 
before the ventral ones. Furbringer (03, p. 432), on the contrary, 
concludes that the dorsal extrabranchials, although almost always 
less strongly developed than the ventral ones, persist longer. 
Gegenbaur says that both dorsal and ventral extrabranchials are 
wanting in most of the Batoidei, for excepting rudiments, only, in 
Rhynchobatus and Trygon, he failed to find them in any of these 
fishes. Parker (’76), also, says that extrabranchials are wanting 
in the Batoidei, so far as he can make out, and he only describes 
ventral ones in Seyllium canicula. Ridewood (’97) has however 
since called attention to the fact that both dorsal and ventral 
extrabranchials were described by Rathke, in 1832, in Raia. 
-aquila and Seyllium canicula, and Foote (’97) has described them 
in Raia erinacea, Raia radiata, Torpedo ocellata and Trygon 
pastinaca. . 
In Chlamydoselachus I find the extrabranchials only slightly de- 
veloped, as Fiirbringer (’03) has already described them. 
In Mustelus (probably laevis) I find the bases of the dorsal 
extrabranchials expanded into relatively large plates which lie 
directly against the dorso-lateral portion of the large venous 
sinus formed by the branchial portion of the vena jugularis, im- 
bedded in the connective tissues surrounding that vein and evi- 
dently specialized, if not developed, in protective relation to the 
vein. They do not reach the inner branchial cartilages by a 
considerable interval, but they are connected with them by the 
connective tissues in which they are imbedded and by a narrow 
band-like muscle related to each extrabranchial. No special liga- 
ments were found binding or connecting the two sets of carti- 
lages. The enlarged bases of the ventral extrabranchials lie 
