the Lower Jaw in Uhizodopsis and RhizoJus. 301 



80 interpreted, llow to fit this bone into the praimaxillary 

 region was to me somewhat puzzling ; and, accordingly, to 

 find it in situ in the head of the fish was an object to be 

 attained, before giving-in adherence to the views usually main- 

 tained regarding it. 



A short time ago my friend Mr. Ward of Longton, to whose 

 liberality in lending specimens from his magnificent collection 

 I am on this, as on other occasions, so largely indebted, sent 

 me a number of unusually good examples of the head of 

 Rhizodopsis preserved in nodules of hard ironstone 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 \-^\- inch 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 oi Megalichthys. Xow, placed between 

 and articulating with the anterior extremities of the right and 

 left maxillte, 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 

 -rV and ^V ir^ch ; 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 

 praimaxill(B 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 fiaked off, leaving behind it a pretty sharp cast witli 

 sutural lines. On close examination a suture is now seen 

 commencing near the posterior extremity of the upper maro-in 

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

 marks off as dentary an element precisely the counterpart in 

 shape of the reputed pra^maxilla. 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. Ihe 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 siipraangidar ; the other two, in 



