Structure of Coccosteus decipiens, Agassiz. 127 



directed forwards under the eye to the premaxilla. To the 

 posterior margin of the maxilla is fixed the jugal or post- 

 maxilla, a triangular plate with posteriorly directed apex, 

 which fills up the space between the maxilla and the lateral 

 part of the body-cuirass. 



So far as I can see, the maxilla of Coccosteus decipiens does 

 not seem to have borne any teeth. But in a specimen from 

 Gamrie in the Edinburgh Museum there is distinct evidence 

 of the presence of both vomerine and palatal teeth. 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 maxilla? and premaxillae. No 

 teeth are, as usual, seen on the maxilla?, 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 mandibu- 

 lar 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 corre- 

 sponding 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 in- 

 wards 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, which would otherwise be sym- 

 physial. This is certainly a very curious circumstance, and 

 one is simply at a loss to imagine 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 Cocco- 

 steus 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 



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