174 



Memoirs of the Indian Museum. 



[Vol.. 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 : — 



Dermal order. 



Dermal disorder. 



1st fart — Subopercular spine. 



2nd fart — Ventral surface of 

 disk. 



yd fart — Dorsal surface of 

 disk. 



Relatively large and tetrafid. 



The space between the pelvic fin and 

 the vent is occupied by seven large 

 plates, a central one surrounded by 

 the six others. The plates are in 

 contact. 



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 bounded 

 externally by an oblique row of 

 plates converging in the direction of 

 the base of the tail. 



Relatively small and irregular. 



The space between the pelvic 

 fins and vent is covered with, 

 about thirty minute plates 

 which are widely separated 

 from one another by naked 

 skin. 



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 . . . . ,, v (M. 



