THE LOWER GREENS AND OF NEW ZEALAND. 327 



time a figure of the type of this species (PL XXI. fig. 4). Although 

 the specimen is much broken, there can be no doubt of its specific 

 identity with those mandibles which have been more recently 

 obtained from the same horizon, and in a more perfect condition. 



Among the fish-remains from the Cambridge phosphatic deposits, 

 examples of /. brevirostris (which can now be definitely determined 

 to be this species) are not uncommon ; and one of the varieties there 

 found agrees very closely, not only with the type specimen from 

 the Gault of Folkestone, but also with the New-Zealand Lower- 

 Greensand example, which forms, in part, tho subject of the present 

 communication. 



As /. brevirostris has never been described, it is proposed here to 

 give the chief peculiarities of the mandible ; and considering the 

 fragmentary character of the type specimen, it is thought better to 

 describe a more perfect example, and afterwards to compare with 

 this, first the original type specimen, and then the one from New 

 Zealand. 



Good evidence of the form of the maxilla and pra?m axilla of /. 

 brevirostris has now been obtained ; but as these parts are not re- 

 quired here for comparison, I do not purpose describing them. 



Ischyodus brevirostris, Agassiz. (Plate XXI. figs. 1-5.) 



The specimen of a right mandible, represented by fig. 1, has been 

 chosen for description on account of its being extremely like Agas- 

 siz's type specimen, and also because it is intermediate in form be- 

 tween two or three extreme varieties, this species being, like others 

 of the family, very variable in form. The general appearance of the 

 jaw will be best understood by a reference to the figure. It is some- 

 what more triangular in outline than is usually the case in this 

 genus, or even in other specimens of the present species. The oral 

 margin (a to d) is seen to be deeply indented anteriorly and pos- 

 teriorly. The front or symphysial margin is slightly convex. The 

 posterior margin is irregular in outline, being always more or less 

 broken. It is evident from the form of these jaws, that they were 

 continually growing at the hinder part, and being thrust forward as 

 the anterior portions were worn away ; from this arrangement it is 

 obvious that at the hinder part the bone would be only imperfectly 

 ossified, and consequently easily broken ; and as a matter of fact 

 the hinder part of these fossils is always more or less broken. 



Viewed from the inner side, as in fig. 1, the oral surface (os) of 

 this mandible is seen to be armed with five of those peculiar denti- 

 nal surfaces which have been sometimes termed tritors, but which 

 will be alluded to in this paper as teeth (fig. 1, abode). The 

 first of these is placed at the end of the beak (a). The dentinal 

 substance of which this is composed is arranged in a series of plates 

 or lamella?, set somewhat obliquely to the anterior border. These 

 lamella? are not seen in fig. 1 ; but in specimens with the outer 

 surface denuded they are seen to extend along near the front mar- 

 gin as in fig. 2. The dentinal substance of which the other four teeth 

 are composed is not arranged in lamella?, but each tooth consists of a 



