36 INFERIOR OOLITE AMMONITES. 



in the young than afterwards, when it takes up nearly the whole whorl, leaving a 

 very narrow terrace ; if the body-chamber be present the inner margin recedes 1 so 

 that the mouth envelops about two-thirds of the preceding whorl. The termination 

 to the body-chamber in the young consists of a lateral process, 2 probably continued 

 until the shell reaches a nearly mature age, and which I previously thought was 



Fig. 1. — Suture-liue of Lioceras opalinum. Natural size. 



continued through life ; but lately the finding of several adult specimens, with the 

 termination quite complete, has shown me conclusively that in adult age the 

 termination is sigmoidal, slightly produced and rounded on the lateral area (the 

 lateral horn having disappeared), and is much produced on the ventral area ; and 

 therefore these characters of the termination are quite in accordance with what 

 we observe in the other species of this genus. 



The suture-line of this species (Fig. 1), which must also be considered as the 

 typical suture line of the genus Lioceras, is taken from a large specimen collected 

 at Haresfield Hill, and is the last suture but one. It will be seen that the points 

 that I have noticed about these sutures (p. 21), and their difference from those of 

 Ludwigia, are here fully borne out, and that the sutures of Lioc. bradfordense 



1 This recession of the inner margin of the body-chamber from the regular line of coil is a 

 fact to be observed in most Ammonites, but is more especially developed in certain genera. In Lioceras 

 it is always very marked, and is present at all ages ; and it must of course follow, for the umbilicus 

 to obtain the small form which it exhibits when the shell is much larger, that, as the body-chamber 

 advances and the inner chambers are formed, the inner margin gradually extends inwards and gradually 

 takes up its proper position on the regular line of coil. As an example ; that part of the inner 

 coil now exposed, just between the inner margin of the mouth and the inner margin of the previous 

 whorl (PI. XIII, fig. 1), would be almost covered by the time the shell had grown another half whorl. 

 The same holds good with specimens of all ages. This is readily seen in any series of specimens, of 

 various ages, of a species belonging to the genus Lioceras which possesses the mouth-border ; and it is 

 a fact which must be especially observed in any determinations of these species, because it is quite 

 possible to find a shell, not having the body-chamber preserved, showing a smaller umbilicus than 

 one nearly half a whorl less in diameter possessing the body-chamber. It is most important to 

 keep this fact well in view when tracing a form from youth to adult age ; and, when measuring the 

 umbilicus, the amount of body-chamber present must always be taken into consideration. If these 

 points be not strictly attended to in a complex genus like Lioceras, with so many forms, it 

 will lead to the confusion not only of varieties but of species. 



2 Quenstedt, ' Cephalopoda,' pi. vii, fig. 10, shows this species with lateral process complete. 



