GRAMMOCERAS SUBSERRODENS. 181 



more acute carina it differs from the above-mentioned species ; and it possesses as 

 a constant feature what Gramm. mactra exhibits only occasionally, namely, a steep 

 inner margin. Another feature peculiar to Gramm. mactra is also seen occasionally 

 in this species, namely, fasciation of the striae producing " bulgings." 



This species has brought Grammoceras to such a state of development that it 

 converges in shape to Lioceras ; and this species especially resembles, and has 

 often been mistaken for, Lioceras opalinum. I have alluded to this before (p. 39). 

 The suture-hne is, of course, the discriminating feature, and it shows that this 

 form is only an involute species of Grammoceras. The suture-line of this species 

 lacks most of the ornamentation visible in that of Lioceras opalinum, has scarcely 

 one defined auxiliary lobe, and has very small accessory lobes in the siphonal and 

 superior lateral saddles. Its lobes are also broader-stemmed, and the inner part 

 is projected more towards the front, that is, higher from the centre-line. At the 

 same time it should be noticed that the suture-line of this species does present a 

 slight development of the suture-line of Grammoceras ; and were we not acquainted 

 with the history of Lioceras, this fact might be twisted into an argument con- 

 cerning the intermediate character of this suture-line between that of Gramm. 

 mactra and that of Zioc. opalinum. The other differences which separate this 

 species from Lioc. opalinum are a sharper ventral area with more prominent 

 carina, ribs less curved on the lateral area, and a rather larger umbilicus. In 

 most respects, however, the characters by which Lioc. opalinum differs from this 

 one are relatively the same as those in which this one differs from Gramm. 

 mactra {Moorei, auct.), and consequently we can exactly see the arguments which 

 were used to unite Lioc. opalinum and Gramm. mactra {Moorei) as one species, 

 this one being considered the connecting link. However, the fact to be remem- 

 bered is that Lioceras appeared in the base of the Falcifei'um-zone, while the 

 ancestor of this species is found in the Jurense-zone as Gramm. striatulimi, a 

 species very unlike and only very distantly related to Lioceras. 



The horizon of this species is in the Moorei-beds {Opalinum- zone). It 

 therefore occurs at the same time as Gramm. mactra, and is common at a time 

 when Lioc. opalinum was only first appearing. When Lioc. opalinum became 

 dominant — that is, in the stage above — this species had disappeared. 



The specific name of this species was applied because of its supposed 

 relationship to Am. serrodens. This species occurs more frequently than Gramm. 

 mactra ; and, though both are found on the same horizon, the two species are 

 seldom associated at the same locality. Grenerally the specimens are casts, and 

 are very poorly preserved ; this is especially the case at Coaley Wood, where the 

 specimens are practically worthless for any purpose of identification. 



I have collected this species from the Opalinum-zo-ne at Ilaresfield Hill 

 (Bed 16, p. 44), Coaley Wood (Bed 6, p. 46), North Nibley (Bed G, p. 46), 



