RAMBLES OF A GEOLOGIST. 289 



for the disproportion between the strength of the plated head- 

 piece and that of the scaly coat was still greater than in the 

 Pterichthys. The occipital star-covered plates are, in some 

 of the larger specimens, fully three-quarters of an inch in thick- 

 ness, whereas the thickness of the delicately-fretted scales rare- 

 ly exceeds a line. 



Why this disproportion between the strength of the arma- 

 ture in different parts of the same fish should have obtained, 

 as in Pterichthys and Asterolepis, or why, while one portion 

 of the animal was strongly armed, another portion should 

 have been left, as in Coccosteus, wholly exposed, cannot of 

 course be determined by the mere geologist. His rocks pre- 

 sent him with but the fact of the disproportion, without ac- 

 counting for it. But the natural history of existing fish, in 

 which, as in the Pimelodi, there may be detected a similar 

 peculiarity of armature, may perhaps throw some light on the 

 mystery. In Hamilton's " Fishes of the Granges" I find but 

 little reference made to the instincts and habits of the ani- 

 mals described : their deep-river haunts lie, in many cases, 

 beyond the reach of observation ; and of the observations ac- 

 tually made, the descriptive naturalist, intent often on mere 

 peculiarities of structure, is not unfrequently too careless. 

 Hamilton describes the habitats of the various Indian species 

 of Pimelodi, whether brackish estuaries, ponds, or rivers, but 

 not their characteristic instincts. Of the Silurus, however, 

 a genus of the same great family, I read elsewhere that some 

 of the species, such as the Silurus giants, being unwieldy in 

 their motions, do not pursue their prey, which consists of 

 small fishes, but lie concealed among the mud, and seize on 

 the chance stragglers that come their way. And of the Pime- 

 lodiis gidio, a little, strongly-helmed fish, with a naked body, 

 I was informed by Mr Duff, on the authority of the gentle- 

 man who had presented the specimens to the Museum, that 

 it burrowed in the holes of muddy banks, from which it shot 



T 



