STICKLEBACKS. 
645 
spinous processes of two vertebras, the posterior on 
those of four. Thus we have as a rule to deal with 
8 interspinal plates at the dorsal edge in front of the 
soft-rayed dorsal tin. In one single case, however, 
Heincke could trace 9 of these interspinal plates, the 
last two each with a distinct, well-developed spinous 
ray, and the third, counting from behind, with a ru- 
Fig. 159. Gcisterosteus aculeatus, trachurus. Natural size. From 
the Frisches Halt, off Kouigsberg. — - ® the posterior, detached part 
of the third dorsal plate (ds. 3 ). After Heincke. 
dimentary spinous ray “in the form of a rigid spine.” 
The Three-spined Stickleback has thus shown at least 
traces of as many as 5 free spinous rays at the dor- 
sal edge. 
If we now observe the arrangement of these 5 
spinous rays, we shall find that each one of the 9 
Fig. 160. ' Gasterosteus aculeatus , trachurus , tetr acanthus, with the 
supernumerary spinous ray set on the penultimate dorsal plate. Natural 
size. From Kiel Bay. — 0 the posterior, detached part of the fourth 
dorsal plate (ds. 4). After Heincke. 
dorsal plates, with the exception of the first, may be 
furnished with a spinous ray, though not simultaneously. 
In order to express the differences that occur in this 
respect, we shall denote each of the interspinal plates 
by an Arabic figure (1 — 9), the plates which in each 
case must be regarded as double, by brackets ( — — ), 
the three ordinary spinous rays by the Roman num- 
erals I — III, the most usual supernumerary ray by IV, 
and the rudimentary and supernumerary ray that has 
only once been observed, by V. The cases (varieties) 
observed by Heincke are then as follows: 
A: in Three-spined Sticklebacks with 3 free spinous rays at 
the dorsal edge. 
a: with the regular number (6) of interspinal plates, the 
forms most typical in this respect (figs. 157 and 158). 
0 0 1 _n_ 0 0 III 
1 — 2 — 3 — 4 — 5 — 6— 0 — 8 — 9 
b: with 7 interspinal plates (the 3rd and 4th detached 
from each other, fig. 159). 
0 0 1 0 II 0 0 III 
1 — 2 — 3 — 4 — 5 — 6 — 0 — 8 — 9 
Fig. 161. Gasterosteus aculeatus, trachurus, tetracanthus, with the 
supernumerary spinous ray set on the penultimate dorsal plate. 
Natural size. From Eckernforde Bay. After Heincke. 
B: in Three-spined Sticklebacks with 4 free spinous rays at 
the dorsal edge. 
c: with a supernumerary spinous ray on the penultimate 
interspinal plate, otherwise as in variety a. 36 speci- 
mens out of 61 showed this arrangement (fig. 161). 
0 0 1 II 0 IV III ' 
1 — 2 — 3 — 4 — 5 — 6 — 0 — 8 — 9 
d: similar to c, but with the 5th and 6th plates detached 
from each other (fig. 160). 16 specimens out of 61. 
0 0 I II 0 0 IV III 
1 — 2 — 3 — 4 — 5 — 6 — 0 — 8 — 9 
e: with a supernumerary spinous ray on the 6th plate and 
with this plate detached from the 5th. 4 specimens 
out of 61. 
0 0 1 II IV 0 0 III 
1 — 2 — 3^4 — 5 — 6 — 0 — 8 — 9 
f: with only 5 interspinal plates, otherwise similar to c. 
2 specimens out of 61. 
0 0 I II 0 IV III 
0 — 2 — 3^4 — 5^~6 — 0 - 8 — 9 
g: with only 6 interspinal plates, otherwise as in d. 1 
specimen out of 61. 
0 0 1 II 0 0 IV 111 
0 — 2 — 3 — 4 — 5 — 6 — 0-8 — 9 
h: with a supernumerary spinous ray on the 4th inter- 
spinal plate, which is detached from the 3rd, the latter, 
