900 THE AMERICAN NATURALIST. [Vor. XXXV, 
The length of the head behind the eyes is also interesting 
here. Tabulating this measurement in the four cases, we find 
that the posterior length is 
(ayin the Kenmare specimen . . . . + + + + 73% 
(6) in the Amenia specimen. . . . + + + = - 78% 
(c) in the Colorado series - . - + - - s+ 3 72% 
GM the SE Paul series o e 0, o 16r ee 
of the total length of the head. In this respect the Amenia 
specimen is very extreme, and the Kenmare specimen agrees 
with the rule for siredons, as determinetl by the Colorado 
series. 
The head is also blunt anteriorly, as in general. The gular 
fold in the Colorado forms, as in siredons generally (see Baird, 
52, Fig. 3), is emarginate, and this is the case in the Kenmare 
specimen, unlike the Amenia one. 
The gills are three on each side. They are vertical out- 
growths of the side of the throat, flattened so as to present a 
dorsal and a ventral margin; the latter, at the base of the gill, is 
continued inward under the branchial bone to form a thin flap, 
the operculum, which covers the gill slit in the floor of the throat 
(see Fig. 6). The gill presents two surfaces — one anterior, 
the other posterior. The former is naked, the latter bears the 
thin triangular plates in which the capillaries are placed which 
expose the blood to the aérating action of the water. The gills 
can be bent by the contraction of a muscle located within 
them, so that the posterior side is parallel to the body, and the 
gill plates are thus covered and protected. 
There are four rows of gill plates. Two of the plates are 
larger and more external. They are on the same level, and are 
followed by two smaller and more internal plates. Each one 
of the plates is a tall triangular and flattened structure. At 
its base a blood vessel can be seen entering on one side. 
Capillaries filled with blood corpuscles can be seen in the 
interior, and running through the gill in the center and near 
the bases of the plate is a large vessel. The blood vessels are 
deeply pigmented, and the surface of the plates is minutely 
spotted with black branching chromatophores. 
