168 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
The lateral hypobranchial artery of the skate was early described 
by Monro (1785, p. 16, Tab. 1, figs. 4, 5), who stated that it was 
connected with all the efferent branchial arteries. Hyrtl (’58, 
p. 16) described it in Raja clavata as coming exclusively from 
the vessels of the second branchial cleft, i. e., the cleft between 
the third and fourth visceral arches; and Parker (’84, p. 61) figured 
it in Raja nasuta as connected with the efferent arteries of this 
cleft and the next posterior. In our experience these differences 
are quite as likely to be individual variations as to be of specific 
importance. 
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The commissural arteries in Raja erinacea are of two kinds, 
dorsal and ventral, of which only the dorsal correspond to the 
commissurals in Carcharias. These dorsal vessels pass through 
the coracobranchial muscle, either in company with the afferent 
branchial arteries of the fourth arch (PI. 1, fig. 2, corns, cl. iv.) , in 
which case they correspond to the fourth commissural arteries in 
Carcharias, or in company with those of the fifth arch, thus repre¬ 
senting the fifth commissurals. A vessel (corns, cl. vi.) whose root 
may possibly represent the sixth commissural is usually present, but 
has never been observed to be connected with the lateral hypo- 
branchial system. In no skate examined by us were both the 
fourth and fifth commissural arteries present, but notwithstanding 
this fact the two sets of vessels were so constant in their relation to 
afferent branchials that their serial homology cannot be doubted. 
The ventral commissural arteries in the skate (PI. 1, fig. 2, 
coms.v.) spring from the lateral hypobrancliials and pass mediad 
between the coracobranchial and the coracohyoid muscles, i. e., lie 
ventral to the coracobranchial muscle instead of dorsal to it. Thev 
may be entirely absent and when present are usually unilateral, 
though a trace of a companion vessel may sometimes be present 
(Fig. 2). They occurred in six of the twelve specimens examined 
by us; in four they were unilateral and associated with dorsal com¬ 
missural arteries on the same side; in a fifth case the ventral artery 
was unilateral but unaccompanied by a dorsal vessel on the same 
side; and in the sixth instance the arteries were bilateral and asso¬ 
ciated with but one dorsal artery. Dorsal and ventral arteries, 
when both are present on a given side, unite near the ventral 
aorta (Fig 2). 
Heretofore dorsal and ventral commissural arteries have not been 
distinguished, but when the figures and descriptions of the earlier 
