POLYPLECTUS DISCOIDES. 219 



and there are seven complete auxiliary lobes. It is noticeable that the forked 

 termination to the superior lateral lobe now becomes conspicuous (fig. 1 /). 

 In the suture at 3| lines side-breadth the follicles of this termination were more 

 divergent ; and the outer one, especially, pointed towards the circumference of 

 the whorl. 



The further development of the suture-line is shown in fig. 1. This is taken 

 from a specimen obtained at White Lackington ; and the breadth of the side where 

 the suture is taken from is 9^ lines. The suture-line, which is given of natural size, 

 shows how the terminal branches of the siphonal lobe have become developed into 

 the accessory tuft. Thus the point a has become very much produced, and has 

 apparently usurped the place which the point b formerly occupied (fig. 4) ; while 

 the points b, c, d, e — the last especially — have all developed, becoming further sepa- 

 rated and more pronounced. The growth of this accessory tuft to the siphonal 

 lobe — that is to say, the great proportionate enlargement of the siphonal lobe — 

 altogether diminishes the importance of the accessory lobe in the siphonal saddle 

 in comparison. The superior lateral lobe is also smaller in comparison, and is 

 now not quite so long as the siphonal lobe. The inferior lateral lobe is still 

 only a little bigger than the accessory lobe, showing that it is not any decrease 

 of the lateral lobes, but inordinate increase of the siphonal lobe, which has caused 

 the difference in the appearance of the suture-line. The auxiliary lobes in this 

 drawing are confessedly incompletely delineated. 



I must now make a few remarks on the way in which the lobe-line differs from 

 d'Orbigny's delineation. The principal point is the size of the accessory lobe in 

 the siphonal saddle. In d'Orbigny's drawing this occupies an entirely subordinate 

 place. While, in my drawing, the accessory lobe hangs down below the upper 

 branches of the accessory tuft of the siphonal lobe, and interposes itself between 

 them and the lower points of the superior lateral lobe, in d'Orbigny's drawing it 

 does not reach the accessory tufts, and there is no room for it to interpose 

 between them and the superior lateral lobe. Again, in my drawing this accessory 

 lobe is nearly as large as the inferior lateral; in d'Orbigny's drawing it is not 

 half the size. It is true that d'Orbigny's figure is taken from a broader side than 

 mine, namely, 15 lines ; but I find that the same characters of the accessory lobe 

 are shown in my specimen at that size. The large accessory lobe is important 

 because it points to the connection of this species with Harpoceras. 



Another point about the suture-line is the peculiar bifurcate appearance of the 

 terminal points of the superior lateral lobe. This character is not shown at all 

 in d'Orbigny's drawing ; and yet it is most conspicuous in my specimen all round 

 the whorl, and gives to these points the appearance of a pair of forceps ; it is 

 also to be seen in the later sutures of the specimens from Milhau (France), as I 

 have stated. 



