Structitre of the Lower Jaw in Ehizodopsis and Ehizodus. 137 



me a number of imusually good examples of the head of 

 Bhizodopsis preserved in nodules of hard h^onstone from the 

 Coal-measures of Fenton in Staffordshire. One of these dis- 

 plays the entire extent of the gape on both sides of the head. 

 Each maxilla measures here 1-jV i^ch in length ; the upper 

 margin is injured; but the lower, bearing one row of small 

 teeth, is quite intact ; the anterior extremity shows the little 

 articular process projecting upwards and forwards as in the 

 similarly shaped maxilla of Megalichthys. Now, placed between 

 and articulating with the anterior extremities of the right and 

 left maxiUa3, while they are joined with each other in the 

 middle line, are two small dentigerous bones forming the front 

 edge of the mouth below the snout. Each of these two bones is 

 nearly as high as long, these measurements being respectively 

 yV and j^^-g- inch ; they are firmly fixed to each other and 

 apparently also to the front of the cranial shield : the teeth, 

 which in this specimen are seen attached to them, resemble 

 those of the maxilla ; but in another example there are traces 

 of others somewhat larger. That we have here the true 

 prwmaxillce is beyond all doubt; some other signification 

 must therefore be found for the bones hitherto considered 

 such. Turning now to the mandible, both rami of which are 

 displayed in the specimen under description, we find that 

 over a considerable area the bony matter of the outer aspect 

 has flaked off, leaving behind it a pretty sharp cast with 

 sutural lines. On close examination a suture is now seen 

 commencing near the posterior extremity of the upper margin 

 of the jaw, and, passing gradually downwards and forwards, 

 marks off as dentary an element precisely the counterpart in 

 shape of the reputed prsemaxilla. The pointed extremity is 

 placed backwards, the enlarged one forwards, the toothed 

 margin upwards. The rest of the outer surface of the mandi- 

 ble is composed of at least three additional bony plates, sepa- 

 rated from each other by sutures which pass obliquely for- 

 wards and upwards. The posterior and largest of these, 

 covering over the articular region of the jaw, may be perhaps 

 equivalent to the angular element, though it also occupies 

 very much the place of a supra-angular; the other two, in 

 front of the latter and below the dentary, may be called 



