LIOCERAS APERTUM. 77 



v-scriptum. PL X, figs. 10, 11, exhibit the largest form of this species in contrast 

 with a young form of Lioc. v-scriptum, figs. 7, 8 ; and on PI. XV the young form 

 of this species is contrasted with Ludwigia rudis and with another form of Lioc. 

 concavum. If this latter (PL XV, fig. 5) be compared with PL X, fig. 10, it will be 

 seen that it looks like a reduced figure thereof ; but if it be compared with 

 what Lioc. apertum is when at the same diameter, it will be found to differ in the 

 smallness of its umbilicus and the character of its ribs. Further investigation, 

 too, will show that PL XV, fig. 3, is exactly the young form of PL X, fig. 10, 

 corresponding in every way, but having (as we should expect, because it possesses 

 the body-chamber) an umbilicus just a fraction larger than that of PL X, fig. 10, at 

 the same diameter. I am thus particular, because it is necessary for me to show, 

 if possible, the actual relationship of the young and old of any form, whether species 

 or variety, and we see that on account of the great apparent similarity of PL XV, 

 fig. 5, to PL X, fig. 10, anyone might be easily misled unless he were most careful. 

 On the other hand, the resemblance that exists between Lioc. apertum and Ludwigia 

 rudis is most striking (especially between figs. 9, 10 and 11, 12, on PL XV) ; and 

 is an instance of that convergence in shape of members of different genetic 

 series, which is recognised as taking place. 1 We have already had to deal with a 

 similar, but not so pronounced, convergence between Ludwigia Murchisonce, var. 

 Baylii and Lioc. bradfordense (p. 23) ; and now we have two forms extremely 

 similar in appearance which it is necessary to put into two different genera ; aDd 

 though Lioc. apertum looks as if it were intermediate between Lioc. v-scriptum and 

 Ludwigia rudis, yet it is in reality nothing of the kind in point of descent. 

 Ludwigia rudis is in no way descended from Lioc. v-scriptum through Lioc. apertum, 

 but comes most probably from Ludwigia costosa? of the Opalinum-zone. Lioc. 

 apertum is, of course, descended from Lioc. elegans (Young) of the Upper Lias, very 

 probably through Lioc. concavum and Lioc. v-scriptum. 



We need not say much about the differences of the sutures of Lioc. apertum 

 and Ludwigia rudis, because the specimens are so small that the smaller auxiliary 

 lobes are merely little denticulations. There are differences however. The 

 sutures of Lioc. apertum (PL A, fig. 17) are in miniature exactly like those which 

 belong to other species of Lioceras ; for instance, Fig. 1, p. 36; PL IV, fig. 8; 

 PL IX, fig. 4 ; and PL A, figs. 10 and 12 — 16. On the other hand, the sutures 

 of Lud. rudis (PL A, fig. 7) are simple, and agree precisely with those of the 

 different species of Ludwigia; for instance, PL A, figs. 1 — 6. They exhibit 

 the disproportion in size between the superior and inferior lateral lobes 



1 See E. Haug, " Ueber die Polymorphidae," ' Neues Jahrbuch fur Mineralogie, &c.,' 1887, Bd. ii, 

 pp. 89 — 163, where convergence between species whose original sources were far wider apart is 

 treated of. 



2 See p. 102. 



