450 INFERIOR OOLITE AMMONITES. 



The above remarks are made because, though the number of species figured is 

 apparently bewildering to one who turns over the plates, it will be seen that 

 by this method of work the number of species to which any given form can 

 possibly belong, or with which it need be compared, is very soon reduced. 

 Instead of looking at the plates the tables should be consulted. Thus by 

 taking external appearance alone the greatest number of species out of the 

 71, which are alike enough to be classed in one column, is sixteen, and 

 this number can be rapidly reduced by proper methods of work. Taking the 

 septa alone, the greatest number of species that are alike is about five ; and of 

 these five no more than two would occupy the same column in the Table of 

 Morphic Equivalents ; that, in fact, only happens in about one case. 



A consideration of the species of Concavum-zone Sonniniae, now figured and 

 described, especially if attention be paid to the method of classification 

 adopted, of which the genealogical table may be said to give a bird's-eye view, will 

 suggest that further generic division of Sonninia will be required at no very distant 

 date. In the first place, it is not difficult to separate all these Sonninise of the 

 Concavum-zone from the true Sonninise, which it may be remarked are not the 

 group of Sowerbyi, but the species allied to propinquans : it is always necessary 

 to work back to propinquans, because that is the type of Bayle's genus. 



It would not be easy to find auy one character applicable to all the species 

 which would separate the whole series from the propinquans-gvoxvo ; but dealt 

 with piecemeal they could be separated, and their characters concisely defined. 

 This, however, must necessarily stand over until propinquans and its allies are 

 figured. Now it is desired to draw attention to this, that the Concavum-zone 

 Sonninise being separated from Sonninia proper lend themselves to further 

 treatment in the way of genera. A few examples will suffice for the present. 

 Thus the series of the quadrifida-stock, and all the species at least of I, A, p. 384, 

 stand out very distinctly, characterised by their symmetrical L ; next, all the series 

 II, A are distinguished by their shallow, narrow-stemmed lobes, and excluding 

 abnormis, they have considerable similarity in ornament and proportions. The 

 ornament and proportions separate them specifically among themselves ; the details 

 of the suture-line separate them generically from quadrifida and allies. Again, 

 II, C, B (p. 389) is a series very marked, on account of the pronounced asymmetry 

 of L and the degeneration in the septa. Here it may be remarked that a further 

 division, of subgeneric value, could be made. The septation separates the whole 

 series generically, but the proportionate amount of costation correlated with any 

 given umbilication separates the series into two parts : a with weak costse — Sonn. 

 spinifera ; j3 with strong costse — Sonn. acanthodes, ptycta, cymatera. The three 

 latter all agree in the strength of this ornamentation, but they are specifically 

 distinguished by the different proportions of tuberculate and costate stages which 



