frcym the Shales of the Northumberland Coal-field. 371 



a substance resembling shagreen, large patches of which fre- 

 quently occur studded all over with it. One such patch has 

 been obtained which measui-ed fifteen inches long and about 

 seven inches wide. On this the Biplodi are comparatively 

 few in number, and are scattered about. But in another 

 patch, of which there are fifty-six square inches, they are very 

 numerous, and are crowded together without order. 



There can be little doubt that these shagreen-like patches 

 are the remains of the skin of some large fish, and that the 

 Diplodi are dermal tubercles in connexion with it, analogous 

 to the spinous tubercles of the Rays. At the same time it 

 must be admitted that it is possible enough that the larger 

 specimens may have clothed the lips or jaws with a spinous 

 pavement resembling in arrangement the oral armature of the 

 Rays or Cestracionts ; or they may have ranged along the 

 back or sides of the body in serial order, as the dermal spines 

 frequently do in the Rays ; or perhaps they may have been 

 scattered here and there among the smaller ones, as is not un- 

 frequently the case with such tubercles. 



Biplodus has usually three recurved spines, two being large, 

 the third quite small ; they stand up from a common, rather 

 deep, rounded or oval base. The two large or lateral spines 

 are ranged side by side ; they are stout, conical, and diver- 

 gent, both being curved from before backwards, and a little 

 compressed in the same direction. The small spine is similar 

 in form, and is placed immediately behind the large ones, at 

 their basal junction ; and in front of them, in a similar posi- 

 tion, there is a large, rounded, depressed tubercle. All the 

 spines are strongly carinated at the sides from the apex to the 

 base ; and in well-developed specimens there are two other 

 ridges, one in front, the other behind, extending downwards 

 for some distance from the apex. 



These are the normal characters of Diplodus ; but it is very 

 variable in form. The spines are not unfrequently found stiff 

 and short, and much bent and divergent ; on the other hand, 

 they often occur much elongated, almost parallel, and compa- 

 ratively slender. The number of spines also varies ; some- 

 times there are only two, sometimes only one. When the 

 latter is the case, the specimen is usually exposed in profile, 

 and the long heel-like projection is well displayed ; when, 

 however, a complete tubercle is buried in the matrix with 

 only one of the lateral spines and its base exposed, the ap- 

 pearance is much the same. A tubercle so seen is repre- 

 sented by M. Agassiz in ' Poissons Fossiles,' vol. iii. tab. 22 h. 



If Diplodus differs much in form, it also varies greatly in 



