TH 1IEE-SP1NED ST ICIvLEBACK . 
without rays. The ventral margin in front of the anal 
tin forms a more or less distinct break for the vent; 
and in front of the vent, at a. greater or less distance 
therefrom, lie the pelvic bones so characteristic of the 
Sticklebacks, or rather their covering bones. Here we 
see the so-called pelvic bones ( propelvis ) distinctly si- 
tuated as osseous growths in the skin, in an homologous 
series with the interhsemal plates mentioned above; 
while in the other Teleosts the pelvic bones, in their 
position and often in their form as well, have proved 
quite as distinctly homologous with the interhgemal 
bones (spines) themselves. This difference is accompa- 
nied by another: in the Sticklebacks the ventral fins 
are attached to the middle (in Gastrcea) or to the an- 
terior part (in Gasterosteus “) of the so-called pelvic 
bones, while in the other Teleosts they articulate with 
the hind extremity of these bones. 
In the Three-spined Stickleback, as in the following 
species, the pelvic bones, with their inner margins unit- 
ed by a suture in the median line of the belly, form 
a compact, triangular plate, pointed behind, which at 
the anterior part of each of its sides sends out a foliate 
disk up the sides of the belly. In the Three-spined 
Stickleback this disk generally meets three of the lateral 
plates of the body on each side, thus forming a com- 
plete pelvic girdle, belonging to the dermoskeleton. The 
anterior margin of the pelvic plate is straight or only 
slightly concave * * * * 6 . The two anterior corners touch the 
interclavicles already mentioned, which in the Three- 
spined Stickleback are comparatively broad — their 
breadth at the middle about equal to the distance be- 
tween them 0 — and so long that the pelvic bones lie 
entirely behind the insertions of the pectoral fins. 
In the typical Three-spined Stickleback the sides 
of the body are almost entirely covered by a corslet 
formed by a row of plates belonging to the lateral line, 
the only naked parts being the belly itself, up to a level 
with the bottom of the insertions of the pectoral fins, 
a narrow strip along the soft-rayed dorsal and the anal 
fins, and the patch in front of each of the pectoral fins. 
641 ) 
Among these lateral plates the middle ones, below or 
just in front of the last spinous ray on the back, are 
the largest (deepest). Forward and backward the plates 
diminish in size; but on the last eight or nine (on the 
peduncle of the tail) we find a raised, median carina, 
following the longitudinal direction of the body, and 
perceptible, though not very distinct, to the twelfth 
plate, counting from behind. This median carina ren- 
ders the breadth (thickness) of the body no less along 
the greater part of the tail than it is in front; but just 
in front of the caudal fin the breadth decreases more or 
less rapidly. The lateral line, which pierces these plates, 
runs from the temporal region parallel to and near the 
back. The sixth plate, counting from in front, is the 
most constant in position, and unites the 4th dorsal 
plate to the top of the ascending disk of the pelvic 
bones, with which disk the 5th and 7th lateral plates 
are also contiguous in most cases. These three lateral 
plates (5 — 7) are also the most persistent, while the 
others may be reduced, attenuated, and obliterated, as 
we have described above. The first to disappear are 
the plates that lie on the posterior part of the ventral 
sides and the anterior part of the sides of the tail, 
between the seventh plate (from in front) and the ca- 
rinated caudal plates: — a form of this description from 
Norway has been named liemigymnus by Collett. Later 
the fish loses both the foremost plates (in front of the 
fifth) and the hindmost, carinated caudal plates, and 
the peduncle of the tail undergoes more and more la- 
teral compression ( gymnurus ). At the same time the 
silvery lustre of the body generally disappears, and the 
back develops more or less distinct, dark spots or trans- 
verse bands, suggesting a retrogression to the juvenile 
characters. 
The head is laterally compressed; the cheeks are 
parallel; but the snout is attenuated in front even on 
the sides. The upper and lower profiles of the head 
meet at fairly equal angles (the upper sloping as much 
as the lower rises), with the exception of a break formed 
by the articulation of the lower jaw. This break is 
“ In the North American Apeltes quadracus the pelvic bones are separated from each other behind, as in the Fifteen-spined Stickle- 
back; the anterior part of these bones (in front of the ventral spines) is short, broad, and continuous, as in the Three-spined Stickleback, 
but without any ascending disk at the lateral margins. The interclavicles are so small that the anterior part of the pelvic bones lies below 
the insertion of the pectoral fins, with the ventral spines just behind it. 
6 In the variety islandicus (according to Sauvage, 1. c., p. 21, pi. I, fig. 8, ci), as in the above-mentioned gymnurus form from Green- 
land, the pelvic plate is both narrow and deeply incised at the anterior margin, just as in the Tcn-spined Stickleback. 
c In some specimens of the above-mentioned gymnurus form from Greenland, however, the interclavicles are as narrow and as widely 
separated at the middle of their length as in the Ten-spined Stickleback. 
