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INFERIOR OOLITE AMMONITES. 



the whorls become broader ; but any further retrogression would cause the sutures 

 to become simpler. 



In extremely retrogressive forms like Sonn. simplex, which have lost every 

 ornament by which the genus was distinguished when at the acme of progressive 

 development, the suture-line becomes almost the only feature to indicate the 

 generic relationship of the species standing by itself. 1 I have, therefore, been at 

 some pains to remove the test of a specimen by means of acid and a file. The 

 suture-line has, no doubt, suffered slightly as regards its minor details ; but its 

 character and its agreement with the sutures of other broacl-whorled Sonnitdse may 

 be seen (PI. LXX, fig. 4). 



In my collection I have associated with Sonn. simplex certain specimens which 

 possess just the same features of coiling, &c, but are a trifle more carinate, and 

 have a sharper upper edge to the inner margin. It is just possible that I may be 

 mistaken in so doing, and that they belong to another genetic series of Sonninia. 

 There are also in my collection specimens more concentrically umbilicate — the 

 central part more slowly coiled and showing a few spines, the only ornament 

 they possess. Such forms are evidently smoother developments of modesta (3, 

 and deserve a separate name. They might be distinguished as sub-simplex. 



All these forms occur at Bradford Abbas ; but there are still some others in 

 the Concavum-hed which might easily be mistaken for simplex. I shall, however, 

 be able to show that they are really the terminals of other genetic series of 

 Sonninia, and are reached by very different routes. There are also forms in the 

 " Sowerbyi-zone " which are terminals of other genetic series. One of these is 

 Sonninia fissilobata (Waagen), 2 to which simplex has much resemblance; but that 

 form really differs in numerous points — the suture-line, the carina, the bend of 

 the striae, and the umbilicus — especially the slower coiling of the inner whorls. 



As it is without ornament, and in order to economise space, only a portion of 

 this very interesting form (Sonn. simplex) has been figured in PI. LXX, fig. 2; but 

 the figure illustrates the characteristic coiling of the umbilicus. An outline of the 

 aperture shows the characters of the ventral area (fig. 3). The suture-line is 

 depicted in fig. 4. 



The submarginata-stock. 



The species of the genus hitherto described — omitting acanthodes — have been 

 arranged in one genetic series with two terminal branches, revirescens and simplex ; 

 though careful work would, as I have pointed out, lead to further division. Those 



1 Without the suture-line the generic position can only be kuown by tracing the genealogy step 

 by step from the acinic forms ; but for this research a very great number of specimens is required. 



2 '/one Am. Sowerbyi,' Geogn. Pal. Beitiiige, Bd. i, Heft 3, pi. xxvii (4), fig. 1, 1867. 



