AMMONOIDEA. 75 



The siphonal side is very narrow and bears a sharp keel. The sutural line at this 

 stage is very simple. The siphonal lobe is broad, but very low ; the first lateral 

 lobe is narrow and short, slightly denticulated at its lower end ; the second lateral 

 lobe is sinailar to the first one, but a little shorter ; then follow five very short and 

 narrow auxiliary lobes, which are not denticulated. The rounded external saddle 

 is low, but very broad, and divided by a short denticulated lobe into two branches 

 of about equal breadth ; all the other saddles are short, broad, and simply rounded. 



At a diameter of about 52 to 54 mm. the sides become slightly tumid and slope 

 towards the siphonal side, so that the greatest thickness of the whorl is close to the 

 umbilicus ; at the same time the siphonal side gradually widens out, and the median 

 keel becomes less sharp, but on either side of it there appears an obtuse, rounded 

 keel. The tubercles on the sides have either entirely disappeared or have become 

 so indistinct that they are only visible in a favourable light. 



With increasing size the siphonal side becomes more and more flattened, and 

 the median keel more and more indistinct until it entirely disappears in full grown 

 specimens. The sagittate cross section of the whorls in young individuals thus 

 gradually changes into an elliptical one in adult specimens. 



The sutural line undergoes very few changes ; the saddles always remain low and 

 much broader than the lobes ; of the latter the first and second become indistinctly 

 denticulated. As the shell grows, saddles and lobes increase slowly in size, and the 

 denticulation of the auxiliary lobes advances further inwards. 



In the largest specimen a remarkable change takes place in the shape of the 

 sutural line, and if the former ones did not exhibit the outline just described, one 

 might feel inclined to consider this specimen to be a different species. The secondary 

 lobe, which divides the external saddle, becomes so deep and broad that it attains 

 nearly the same size as the first lateral one. On the other hand, the two branches 

 of the external saddle, as well as the first lateral one, become very narrow and high. 

 At the first glance this sutural line shows a very different form from that of smaller 

 specimens, but on tracing it back, it may be noticed how, with the deepening of the 

 secondary lobe, the two branches of the external saddle and the first lateral one gra- 

 dually stretch, while becoming narrower. 



Full grown specimens show hardly any traces of the radiating tubercles, unless 

 perhaps a slight roughness of the shell betrays their previous existence. 



Most of the specimens have the shell almost entirely preserved ; it is very thin, 

 barely 1 mm. in thickness, and perfectly smooth except for numerous strise of growth ; 

 beginning from the sutural line these striae first bend strongly towards the aperture 

 till about two thirds of the height is reached ; then they curve backwards slightly 

 bat immediately turn to the front, and forming a short projection on^the siphonal side. 

 If from the run of the striee of growth a conclusion may be drawn as to the shape of 

 the aperture, it might be supposed, that the latter was provided with two short but 

 broad, rounded lateral ears, separated by a shallow and narrow sinus from a short, 

 gquare, siphonal process. 



Locality and straligraphical position. — Mazar Drik. 



fLemarks.^-This, species is easily distinguised by its sutural line ; the external 



L % 



