112 MESSES. HANCOCK AND ATTHEY ON 



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 divergent, 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 junc- 

 tion ; and in front of them, in a similar position, is a large, 

 rounded, depressed tubercle. All the spines are strongly cari- 

 nated at the sides from the apex to the base ; and in well deve- 

 loped 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 Dijjiodus ; 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 compara- 

 tively slender. The number of spines also varies ; sometimes 

 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 com- 

 plete tubercle is buried in the matrix with only one of the lateral 

 spines and its base exposed, the appearance is much the same. 

 A tubercle so seen is represented by M. Agassiz in "Poissons 

 Fossiles," vol. iii., tab. 22 b, fig. 5. 



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

 The largest are f ths of an inch from the base to the apex of the 

 large or lateral spines ; the smallest, measured in the same way, 

 are not more than iVth of an inch in extent. Between the two 

 extremes, tubercles of every size occur. Now the smaller indi- 

 viduals, which are by far the most numerous, agree very well 

 with Diplodus minutus of Agassiz, so far as the imperfect speci- 

 mens described and figured by that author permit a comparison. 

 M. Agassiz says he was not able to discern the median cone ; 

 but this is not to be wondered at, for none of his figures repre- 

 sents the base entire. 



moT^.-Dittodus divergens, Aganodus apicalis, Agcmodus undatus, 



