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INFERIOR OOLITE AMMONITES. 



but the inner part of whorl steeply sloped. Inclusion very slight, about one- 

 fifth. Umbilicus rather depressed, mostly costate, but the central part with a few 

 rudimentary spines. Suture-line with a narrow-stemmed superior lateral lobe of 

 which the lateral lobules are fairly balanced. 



This remarkable and very distinct form is singularly like Asteroceras obtusum 

 of the Lower Lias — hence the name I have given to it, although the term is 

 equally applicable to its general dimensions. In outward aspect, even to the 

 sulcate-carinate ventral area, it simulates Ast. obtusum so closely that it is only 

 to be distinguished therefrom by having reclining and not inclining costse ; but 

 its suture-line at once tells a different tale ; in that respect, there is not the least 

 similarity between the two forms. 



Solium in obtusiformis is, in nearly all respects, a morphological equivalent of 

 Aster, obtusum. The spinous central whorls of the latter show it to be a costate 

 retrogression from an ancestor which belonged to the spinous stage ; the spinous 

 inner whorls of the former point to exactly the same facts. Yet the analogy is 

 not (juite complete, because the distant ribs which render obtusiformis so like 

 obtusum are due to a renewed progressive development of the costate stage, but 

 the distant ribs of obtusum are due to less-marked retrogression of the costate 

 stage. 



The renewal of progressive development in obtusiformis is a very remarkable 

 feature. The larger-sized ribs are noticeable, but not to any great extent, in the 

 adult ptycta ; the same feature appears more marked and rather earlier in cymatera; 

 but in obtusiformis this development is a pronounced feature of the early adolescent 

 stage — in fact, the feature appears nearly two whorls earlier in obtusiformis than in 

 ptycta. 



It may be noticed that a morphologically-equivalent development of ribs is 

 shown in the dominans- stock (revirescens) . In the present case, however, renewed 

 progressive development is even more pronounced, for it is correlated with a 

 noticeable inflation of the whorls and with sulcation of the ventral area, both 

 these being progressive features which had presumably accompanied the 

 progressive development of the iS^vm-wv'ft-ancestors. 



Sonninia obtusiformis has every appearance of being anything but a decadent 

 fori ii ; yet I cannot at present point to any species which might be regarded as its 

 descendant. The branch-stock comes to an end with this species so far as I am 

 aware ; yet it is not a terminal like simplex and suJ> striata. 



Of the species of Sonninia which I have described marginata is the only one 

 which obtusiformis is at all like; but the present fossil has reclining ribs nearly 

 twice as distant and certainly twice as large, is practically without any spines, 

 has considerably inflated whorls, and a markedly sulcate ventral area. 



This fossil is unique. It was obtained by my father probably as many as 



