ON PLEUEODUS RANKINII. 409 



immediately in front of and in connexion with the body, but it 

 is so much distorted that the form cannot be determined : it is 

 about five-eighths of an inch long. No bones are distinguishable ; 

 but the substance is here a little thickened, indicative of the car- 

 tilaginous remains of the cranium ; nor is there anywhere in the 

 body the least appearance of bones, the skeleton undoubtedly 

 having been cartilaginous throughout. The teeth (b) lie within 

 the area of the head, in a disturbed condition, some with the 

 crown uppermost, others with it downwards. They do not seem 

 to have been numerous, but are so obscured that the exact num- 

 ber cannot be ascertained. In the detached head, however, ten 

 or a dozen can be counted ; but there is no certainty that the 

 number may not have been greater ; indeed, it is probable that 

 some have been removed with the counter slab. 



The body suddenly widens immediately behind the head, the 

 width being considerably increased by the thoracic expansions 

 (c, c), which extend about half-way down and appear to have 

 had their margins pointed ; thence it tapers backwards, and soon 

 dies out, there being no definite indication of the form of this 

 portion ; and, as has been already stated, there is no trace of 

 the tail. The spine (d) is situated a little behind the thoracic 

 expansions ; consequently it is nearer the posterior than the 

 anterior extremity. It projects from the dorsal margin, and is 

 inclined backwards, apparently in its natural position, marking 

 the situation of the dorsal fin ; but no traces of this remain. 

 About two-thirds of the spine is in a good state of preservation, 

 the other third being well and sharply defined in cast ; it is 

 straight and stout in proportion to its length, and tapers some- 

 what abruptly to a sharp point ; it is compressed laterally, with 

 the anterior margin thicker than the posterior; the surface is 

 coarsely and irregularly grooved and ridged longitudinally ; it 

 measures five-eighths of an inch in length, and is at the thickest 

 part one -eighth of an inch wide. 



Shagreen covers the whole of the specimen, defining its ex- 

 tent and form, though, as already noticed, with no great preci- 

 sion, as the margins are much blurred by the displacement of 

 the shagreen-tubercles ; but, notwithstanding this disturbance, 



