222 INFERIOR OOLITE AMMONITES. 



possible, with a very extended series of specimens, to trace tlie gradual development 

 of one so-called species into another, so that many of the lines of division between 

 the forms named are more or less arbitrary. Such, for instance, is the case between 

 Gramm. toarcense and Gramm. striatulum, — between Grmnm. mactra and Gramm. 

 subserrodens, — between Gramm. fiuitans, aalense, and leurum, — and from Gramm. 

 quadratum to Muelleri on the one hand and to Gotteswoldids on the other. At the 

 same time, although the boundaries between the species may be bridged over more 

 or less, it is certainly expedient to retain names for the different stages of 

 development in any genetic series, provided that these steps possess peculiar 

 features sufficiently distinct and recognisable. So far as more intermediate 

 specimens are concerned, it may be worth while to consider a suggestion made to 

 me by Mr. H. B. Woodward, F.G.S., namely, to use a double name for intermediate 

 forms ; thus, Gramm. madrasuhserrodens would mean the form intermediate 

 between these two species. 



One fact of particular interest, which the Grammocerata bring prominently to 

 our notice, is that Ammonite- species exhibit a tendency to vary in several 

 directions ; and, further, that one of these variations is successful and develops 

 into a well-marked form, which is deemed worthy of another name. It is true 

 that the variations to be observed in a species are slight characters which can 

 very often only be detected on extremely good specimens, but this is obviously a 

 necessity of the case. If the characters were more strongly marked the form 

 might receive a distinct name. 



Suppose we take Gh^amm. mactra as an example. Its tendency is towards 

 finer ribbing, but in one case this is accompanied by a decay of the carina 

 (PI. XXXI, figs. 3, 4) ; in another case by a sharpening of the carina, a fasciation 

 of ribbing, and a slight inner margin (PI. XXXI, figs. 1,2); in a third case by a 

 sharpening of the carina, a pronounced inner margin, and more compressed sides. 

 Now it is the third case which seems to be continued into Gramm. subserrodens. 



The case of Gramm. aalense is much the same. There are five slightly different 

 forms of this species figured. One of these (PI. XXXII, figs. 7, 8) is the form 

 which is continued into Gramm. leurum. 



Concluding Remarks on the Family Hildoceratidx. 



By considering the base of the Opalinum-zone as the base of the Inferior 

 Oolite, the species and genera which comprise the family Hildoceratidx are divided 

 most unnaturally into two sections — Oolitic and Liassic. In consequence of the 

 manner in which the Jurense- and the Opalinum-zone are bound together by the 

 genera Haugia and Grammoceras, I have found it necessary to trespass beyond 



