Memoirs of the Indian Museum. 
174 
[Voi,. II, 
specimen in which this spine is large and tetrafid the ossicles are arranged in a 
definite pattern. The full characters may be defined in the following way : — 
Character. 
Dermal order. 
Dermal disorder. 
is^ part — Subopercular spine. 
Relatively large and tetrafid. 
Relatively small and irregular. 
2nd part — Ventral surface of 
The space between the pelvic fin and 
The space between the pelvic 
disk. 
the vent is occupied by seven large 
fins and vent is covered with 
plates, a central one surrounded by 
the six others. The plates are in 
contact. 
about thirty minute plates 
which are widely separated 
from one another by naked 
skin. 
yd part — Dorsal surface of 
disk. 
On the dorsal surface is a median row 
of four or five large plates. 
On either side of the median row is an 
area of naked skin which is boimded 
externally by an oblique row of 
plates converging in the direction of 
the base of the tail. 
There is no area of naked skin 
on either side of the median 
row ; the whole dorsum is 
covered irregularly with plates. 
Thus the character may be divided into three parts. Of these the first is nearly 
constant, the second is constant, but the third is less constant. The aberration shown 
by those individuals which are not quite true to type is always in the direction of the 
opposite character. For example, we shall see in dealing with the types and indivi- 
duals, that among those which possess small irregular subopercular spines some show 
a slight tendency in the direction of the tetrafid type. 
The contrast between the ventral surfaces of the two opposite types is most 
striking to the eye and is clearly visible in the photograph. It may be described at 
greater length. In the orderly type besides the seven plates already mentioned there 
is a pair of large plates between the bases of the pelvic fins. In front of these are 
six other plates, a large central one surrounded by five others. In the disorderly 
type these plates are represented by numerous small platelets distant from one another 
and arranged irregularly. Beneath the lower jaw of all the specimens is a semilunar 
area covered with little pits containing minute tentacles. In the orderly type this 
area is marked off from the rest of the disk by a row of four large plates, which 
are in contact with one another. In the other type these plates are absent ; in one or 
two cases, however, they are represented by four minute platelets widely separated 
by naked skin, which are only visible with the aid of a lens. 
Although there are two types of dermal armature, the orderly and disorderly, we 
have seen that there are three types of disk, a narrow, a medium and a broad. The 
characters occur independently of one another so that six types of fish are possible — 
1. Orderly and narrow . . . , . . Type x. 
2. Orderly and medium . . . . . . ,, w. 
3. Orderly and broad . . . . . . ,, z {M . triangularis). 
4. Disorderly and narrow . . . . ,, {M. lutea)- 
