from the, Shales of the Northumberland Coal-field. 373 



is in part a natural cavity, frequently seen in sections. A. un- 

 datus (pi. 10) is a lateral section of a single minute spine of 

 the same variety of Diplodus^ somewhat abnormal in form. 

 There is no difference of importance in the minute structure, 

 and it exhibits in a most distinct manner the numerous con- 

 centric layers of dentine mentioned by M. Agassiz as charac- 

 teristic oi Diplodus (vol. iii. p. 209). 



Diplodus has supplied Prof. Owen with still another generic 

 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 evident 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 spc" 

 cimen the peripheral dentine occasionally varies considerably at 

 different parts of the circumference, as may be seen on making 

 a transverse section of the spines ; and as they are com-> 

 pressed, 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 pro- 

 portion to the dentinal wall would depend upon the degree of 

 eccentricity of the section. 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 

 * Trans, of the Manchester Geol. Soc. vol. i. pi. 5. fig. 17, 



