RESUME. 125 



Fig. 20. V-shaped, from a specimen of Lioceras concavum variety v-scriptum. 



Fig. 21. V-shaped; two examples taken from different places on the same 

 specimen, the larger showing the commencement of the change in shape. 



Fig. 30. Sigmoidal, with long outer arc, taken from the specimen of Hildoceras 

 bifrons which furnished the suture-line fig. 29 ; v = middle of ventral lobe, V =z of 

 superior lateral lobe, Z" = of inferior lateral lobe, w= junction of ventral and 

 lateral areas. The dotted line through the genicula marks the place of the 

 longitudinal furrow. 



Fig. 45. Subarcuate, taken from a specimen of Grammoceras striatulum ; v= 

 ventral lobe, V superior lateral, I" inferior lateral. This kind of ribbing has usually 

 been called sigmoidal, as well as that shown in fig. 18 ; but I think it desirable 

 to keep a separate term for ribs which have such a long ventral sweep, and have 

 the lateral portion fairly straight. The inside point should have been very slightly 

 turned towards the front. 



I now come to the second task of which I purposed to treat in this resume, 

 and which relates chiefly to the vexed subject of classification. In illustration of 

 the subject it becomes necessary for me to demonstrate the descent of the various 

 species as far as possible ; and it also becomes necessary for me to make a few 

 corrections in accordance with advancing knowledge. It is some four years since 

 I wrote the first part of this Monograph, and during that time many causes have 

 increased the information in general, and my own knowledge in particular, con- 

 cerning Ammonites. In the first place, I have been able to examine the strata 

 known as " The Cotteswold Sands " and " Cephalopoda-bed," and have therefrom 

 obtained a large number of species, few of which I formerly possessed, and which, 

 since they belong more exclusively to a somewhat confined series, have given me 

 more insight into the characters of that series than I was able to obtain in any 

 other way. Furthermore, I have had the opportunity of being able to inspect 

 several important collections of Ammonites, which have been sent to me by the 

 great kindness of different correspondents for examination at my leisure. The 

 further inspection of some museum-collections has been of service to me ; but the 

 limited time at my disposal, the too frequent paucity of duplicates, the absence of 

 unrecognised species, and sometimes the fact that the specimens cannot leave the 

 building, necessarily restrict the value of such work. 



At page 15 of this Monograph I put forward a scheme of grouping. I tried 

 to accomplish the somewhat difficult task of making an old system work in a new 

 fashion with the fewest possible alterations. I also had, as it happens, too great a 

 desire to preserve the names used by other authors, though, while in most instances 



