436 INFERIOR OOLITE AMMONITES. 



show as plainly as it should for tracing, and probably some of the minor details 

 have suffered. The main details are probably correct, and they show, first, great 

 length of lobe, rather abbreviated lateral lobules, possibly due to gerontic degene- 

 ration, and an ecto-brachysceles terminal lobule. The latter feature accompanying 

 presumed septal degeneration is remarkable, because it is a return to what obtains 

 in a biologically-earlier form, Zurcheria. The septal margin of an immature 

 specimen which, as far as it goes, is nearly an exact counterpart of the type, 

 except in being rather more costate and rather more spinous, is shown in fig. 2 

 of PI. XCIV (see also outline of L, fig. 55, p. 435). It agrees with that of 

 the type in remarkable length of lobe, and there is an incipient ecto-brachy- 

 sceles character to the terminal lobule of L. It may be described as follows : — 

 Suture-line with long and very ornate lobes, the side being almost occupied by a 

 complete network. Siphonal lobe with bifurcate terminal branches and three 

 lateral accessory lobules ; L tetradactyloid because the outer lobule is double, 

 being divided nearly to its base, the terminal lobule as long as the main stem of 

 the lobe, intra-axial, nearly isosceloid, the inner lobule about equal in size to one 

 branch of the outer lobule. 



It is presumed that this is the true form of the suture-line before septal 

 degeneration takes place. On this presumption the present position of the species 

 is based. Further, the septal differences now disclosed show that under the name 

 Bonn, dominans a polygenetic series of morphic equivalents had been collected. 

 It was pointed out at the time (p. 323) that the forms could be distinguished 

 externally, and they were grouped as varieties a, (5, y, S. Now that these external 

 differences are shown to be connected with distinct septal details, it has become 

 necessary to rearrange these forms. From this association must be removed the 

 specimen figured in PI. LXIX, 1 as well as the forms /3 2 and <S, 3 p. 323, and the 

 immature specimen, Plate LXVII, figs. 1, 2/ There remains then as the type 

 Sonn. dominans the specimen figured in PI. LXII ; and to this may be added, as 

 an immature, slightly more ornate example, the specimen shown in PI. XCIV, 

 figs. 1 and 2, and PI. XCV, fig. 1. Concerning other specimens described as 

 " dominans " or " near dominans " (PI. LXVII) see later (p. 438). 



A figure of the immature Sonn. dominans has been placed on the same plate as 

 that of Sonn. dominatrix, to facilitate comparison. It will be seen that until 

 about the last half-whorl they are externally almost exactly alike, and in the last 

 half-whorl of the figures there is no very great difference in ornament. At any 

 rate, the difference was not enough to induce the suspicion of such remarkable 



1 See Sonn. dominica (p. 410). 



2 See Sonn. dominata (p. 408). 



3 See Sonn. dominatrix (p. 392). 



4 See Sonninia, sp. (p. 438). 



