114 MKSSRS. HANOOOK AND ATTHEY ON 



Diplodus has supplied Professor Owen with still another ge- 

 neric form, which is the fourth based upon this variable fossil. 

 Ochlodus (pi. 5) is nothing more than one of the large varieties 

 of this dermal tubercle, crushed laterally, a variety, probably, 

 having originally one of the large spines smaller than the other. 

 A figure of such a tubercle is given by Mr. Binney in the paper 

 before quoted.* From the representation of Ochlodus it is evi- 

 dent that the specimen has been crushed : the dentinal walls are 

 cracked in several places, the upper wall has been forced in upon 

 the osteo-dentine of the pulp-cavity, and the continuity of the 

 tissue of the spines has been severed ; the osteo-dentine of the 

 pulp-cavity has, in a great measure, been displaced, and the base 

 shattered to fragments. All these appearances are shown in a 

 section now before us, which was made of a specimen crushed 

 laterally or a little diagonally, and which closely resembles in 

 size and contour Ochlodus. It is evident, too, that much of the 

 fractured base in this genus, and also a considerable portion of 

 the two smaller spines, have been removed in making the section. 



The thickness of the dentine and the size of the pulp-cavity 

 are very variable features in Diplodus. Even in the same spe- 

 cimen the peripheral dentine occasionally varies considerably at 

 difi'erent parts of the circumference, as may be seen on making a 

 transverse section of the spines ; and as they are compressed, as 

 we have stated above, the relative size of the pulp-cavity varies 

 with the plane of the section. This is one source of variation ; 

 but were the pulp-cavity quite cylindrical, or rather circular in 

 transverse section, its apparent relative proportion to the denti- 

 nal wall would depend upon the degree of eccentricity of the sec- 

 tion. The pulp-cavity is consequently found to vary extremely 

 in size in Diplodus. In the crushed specimen we have spoken 

 of, this cavity is quite as large as it is represented in the figure 

 of Ochlodus ; and, again, in other specimens it is no larger than 

 we see it in the figure of the so-called Pternodus productus. 



The acute points represented in the section of Ochlodus are 

 not the apices of the spines, as believed by Prof. Owen ; the true 

 apices have all been removed in making the section. These 



* "Transactions of the Manchester Geologieal Society," vol. I., plate V., fi?. 17. 



