Oil the Structure of Coccosteus decipieiis, Arjassiz. 21o 



of the presence of both vomerine and palatal tectli. The 

 specimen lies on its back, giving a beautiful view of the 

 ventral cuirass, in front of which are the two rami of the 

 mandible converging to meet each other anteriorly, while 

 external to and in front of them the upper margin of the oral 

 cleft is seen formed by the maxilhe and premaxilla;. No 

 teeth are, as usual, seen on the maxilhe, but internal to them 

 and between them and the contiguous mandibular ramus is 

 seen a row of conical teeth, evidently placed on the edge of a 

 palatal or palato-pterygoid bone, which I have not yet seen 

 in its entirety. Also in front of the meeting of the mandi- 

 bular rami and behind the premaxillary and ethmoidal region 

 is a clump of five conical teeth, clearly vomerine in position 

 at all events. It is also clear that the whole of the dentition 

 of the front of the mouth is not here exposed, as the clump 

 referred to is on the left side of the middle line, and the 

 corresponding space on the right side is covered by the 

 anterior extremity of the corresponding mandible. 



The bone representing the mandible is well known from 

 the description of Hugh Miller. It is an elongated, vertically- 

 flattened plate (Fig. 1, mn), broader behind than in front, 

 with rounded posterior extremity, slightly sigmoid contour 

 when seen from the side, and near the anterior extremity 

 sharply bent inwards towards its fellow. It is remarkable 

 for having two sets of conical teeth, one consisting of a row 

 of about half-a-dozen being situated about the middle of the 

 upper margin of the bone, while another row of about the 

 same number occupies the vertical anterior margin, whicli 

 would otherwise be syiiiphysial. This is certainly a very 

 curious circumstance, and one is simply at a loss to imaoine 

 of what use teeth could be in such a situation, or how they 

 worked. It was indeed the position of these peculiar 

 symphysial teeth that led Hugh Miller originally to compare 

 the working of the jaws of Coccosteus with those of an 

 Arthropod (2, 1st ed., p. 57; see also footnote in 4th and 

 subsequent editions). 



There are no traces of ossified internal cranial bones, of 

 hyoid or of branchial arches ; consequently these parts must 

 have been entirely cartilaginous. I may mention that the 



