320 NOTES ON THE STRUCTURE AND RELATIONS 



not very unfrequent in specimens of our own species, which 

 are known familiarly as double or buck-teeth. But, to con- 

 clude this part of my subject, what I have to state regarding 

 the jaws of the earlier Coelacanths is simply this, — that as the 

 alligators are characterized generically by cavities in their up- 

 per jaws for the reception of their fourth teeth anteriorly on 

 both sides, and as the Lepidostei are characterized generically 

 by cavities in their intermaxillaries for the reception of the 

 anterior projecting teeth of the lower jaw, some of the earlier 

 Ccelacanths, such as Asterolepis and Rhizodus, were charac- 

 terized by a line of cavities in both under and upper jaws, 

 which received, when they shut their mouths, all their reptile 

 teeth ; and further, that there occun'ed certain irregularities in 

 their dentition, somewhat similar to those which occur in the 

 dentition of Lepidosteus, but still more strongly marked, which, 

 though they impart a marked singularity of aspect to the spe- 

 cimens in which they occur, were not specific peculiarities, but 

 merely individual ones. I may be permitted one other re- 

 mark. Some of the reptile teeth of the Carboniferous Rhizodi, 

 as shown in specimens from Gilmerton and Burdiehouse, had 

 great apparent depth of socket ; and a reptilian relationship 

 has been deduced, I doubt not legitimately, from this pecu- 

 liarity. More immediately, however, this depth of socket 

 seems to be a consequence of the depth of pit required by the 

 opposite tooth for its reception, and which is always great in 

 proportion to the length of the tooth above the edge of the 

 jaw. The long sword requires a deep scabbard to sheathe it 

 in ; and as the lower part of the reptile teeth of the one jaw 

 usually forms the one side of the pits in which the reptile 

 teeth of the other jaw were sheathed, their depth of apparent 

 socket is proportioned to the depth of the sheath. 



There was one respect in which the Coccosteus differed 

 from all the other ganoids of the Old Red Sandstone yet 

 known. While its head was covered with a strong osseous 



