FISH OF THE CARBONIFEROUS PERIOD. 25 



longer than wide ; anterior end narrow subtruncate ; surface 

 obliquely crossed by nine or ten thick, flat, imbricating ridges, 

 varying from one line to half a line wide ; they run nearly 

 straight, but each has got an abrupt angular bend about the 

 middle, which makes the posterior half of each edge seem about 

 half a line further out than the anterior half; each imbrication 

 has one, or rarely two rows of large equidistant puncta or 

 small pits. 



This differs from the P. Jonesii (Ag.) in its very narrow elon- 

 gate form, more numerous flat imbricating ridges, the row of 

 notch-like curves one in the edge of each, and the regular rows 

 of great puncta. The specimen described is imperfect, but seems 

 to have been about 1^ inch long and 4 lines wide. 



From the upper black beds of limestone in Derbyshire. 



(Col. University of Cambridge.) 



• Chirodus (M^Coy), n. g. 



(Etym. %efc/o, manus, and 6Bov<i, dens.) 



Gen. Char. General form of Ceratodus, that is more or less fan- 

 shaped, thick, flattened, with the anterior broad margin deeply 

 divided into lobes ; but the inner, nearly straight margin has a 

 small, recurved, thumb-like lobe projecting nearly at right 

 angles from the middle of its length (preventing the mesial 

 junction of the tritors of each side of the jaw) ; the inner mar- 

 ginal lobe is the longest ; surface minutely punctured. 



The only specimen which has occurred to me of this genus 

 presents only two lobes in the anterior margin, but as the outer 

 edge is imperfect there may have been another lobe, but I think 

 not more. I should have referred the tooth to the Permian ge- 

 nus Ceratodus, but that the inner margin (which in Ceratodus is 

 straight to fit the similar edge of the tooth on the other side of 

 the jaw) has got a small lobe projecting horizontally inwards from 

 its middle, which would prevent such a union ; or if it be viewed 

 as possibly the outer margin, we would have the equally singular 

 characters not only of so great a disparity in size between two 

 adjacent lobes, but the principal marginal lobes would increase in 

 size from within outwards, which would be contrary to all ana- 

 logy. Hence, independent of the geological importance of not 

 extending unnecessarily the vertical range of a genus, we find it 

 zoologically impossible to group together teeth so differently con- 

 structed that they could not be similarly arranged in the mouth. 

 I have named the genus from the general resemblance to a hand, 

 or still more to the foot of a Chirotherium in miniature. 



