Variations in Afferent Branchial Arteries of the Skate. 237 
as far as recorded, are confined to the species Raia circularis, Couch, and 
occur in the male sex only. 
The occurrence of the abnormalities in Raia circuiaris only, appears 
worthy of remark, because that species is not the one most frequently 
delivered for use in the class. Four species only, as far as the writer’s 
experience goes, have occurred among the specimens delivered to the 
department of Zoology during a period of six years, and of the four, 
Raia radiata, Donovan, is by far the most common. aia circularis, it is 
true, also occurs with considerable frequency, and on some occasions arrives 
at being the most numerous species in lots of from twenty to forty specimens 
delivered at one time. ava batis, Linn., might be expected to be represented 
by from three to five individuals among such numbers, while £. clavata, Linn., 
occurs only very rarely. In order to give an exact record of the relative 
frequency of the four species, the numbers of each were counted on six days 
during the autumn term of 1920, and the result, out of a total of 184, was— 
R. radiata, 132; R. circularis, 36; R&. batis, 15; and #. clavata, 1. 
No very exact numbers can be given to indicate the frequency of the 
abnormalities in question, for the lots above enumerated were not dissected 
for afferent branchials. During the week before these enumerations were 
begun, two abnormal specimens (Figs. 2 and 7) were obtained, these two 
being the only variations of the kind observed among about 170 specimens 
dissected for afferent branchial vessels during the week. Among these 
170 or so specimens R. circularis was well represented, and two or three 
specimens of ZR. clavata also occurred. — 
One other fact may deserve passing mention, and that is that the-gills 
of the specimens represented by Figs. 1 and 4 were infested by the not 
uncommon Copepod parasite, Charopinus dubius (T. Scott). In Fig. 1 the 
gills on both sides were attacked. 
Regarding the actual abnormalities a few words only are needed in 
explanation of the accompanying figures. In the majority of typical 
Elasmobranchs the normal condition presents five afferent branchial vessels 
on each side, running respectively on to the hyoid arch and the first four 
branchial arches. The method of origin of the afferent vessels from the 
ventral aorta—whether fused or independent—varies according to the genus. 
In Rava the first two afferent vessels (counting from the anterior end) arise 
normally as branches from the anterior innominate, while the last three are 
branches from the posterior innominate. Both anterior and_ posterior 
innominates are paired vessels arising directly from the ventral aorta. In 
none of the abnormal cases is the total number of five afferent vessels 
departed from, the exceptional conditions being usually occasioned by the 
