Specimen o/'Pleurodus Rankinii. 251 



ment of the shagreen-tubercles ; but, notwithstanding this 

 disturbance, towards the margins in many pLaces considerable 

 patches of them lie in their natural order, particularly on the 

 right thoracic expansion, on a large portion of which the 

 shagreen is entire. The tubercles are very minute, requiring 

 a powerful lens to exhibit them, and the lower powers of the 

 microscope to display their characters. They are many-sided, 

 irregularly formed bodies, closely titting together like mosaic 

 work ; the surface is a little raised and beset witli irregular 

 rugge. This is the appearance presented where the shagreen 

 is undisturbed ; but it is doubtful whether it may not be the 

 under sm-face that is presented to view. In places where the 

 tubercles are scattered numerous shining bodies are observed ; 

 these are about the same size as the tubercles, and, like them, 

 are irregular in form, but are more gibbose, and have a ridge 

 or two on the surface, which are produced into points at one 

 of the sides. From analogy we might suppose that these 

 bodies exhibit the upper surface of the shagreen-tubercles ; 

 but fm'ther observations are required to determine this point. 



The teeth are boss-like in form, somewhat elongated and 

 ridged or carinated along the longer axis ; the sides are con- 

 siderably expanded in the centre, the expansions dying out 

 towards the ends of the tooth ; usually the expansion is more 

 produced on one side than on the other, and the ridge inclined to 

 the opposite side. The expansions are frequently transversely 

 ridged or plaited, and sometimes tuberculated. The central 

 ridge or carina of the crown is arched in the long axis of the 

 tooth, following the curvature of the surface, and is frequently 

 reduced, as if by wear. The whole surface of the tooth, as 

 well as the lateral expansions, is covered with a thick brownish- 

 white enamel, and is coarsely punctate, the punctations being 

 most conspicuous when the enamel is worn off. The tooth 

 measures two tenths of an inch in length : a variety, however, 

 foui* or five of which have occurred at Kenton, is twice that 

 size ; but it is more oblique than the small form, has no coronal 

 carina, and is broad and rounded on the upper surface ; in all 

 other respects it agrees with the small and usual form. It is 

 quite possible that these large teeth may belong to another 

 species. 



From the above description it will be perceived that Pleu- 

 rodus is a not very distant ally, as we have already stated, of 

 Wodnikaj of the Magnesian Limestone, the relationship 

 being seen in the characters of the spine and shagreen, and 

 particularly in the form of the teeth : in both genera they 

 have the same boss-like, carinated crown, with expanded la- 

 teral margins, more or less ridged or crenate ; in both, too, 



18* 



