1870.] 25 [Hyatt. 



genetic connection is traceable from species to species, and these 

 species agreeing quite closely, even as regards the two most widely 

 separable forms in the proportions and outline of their septa. The 

 planicostan abdomen is a reversionary feature, occurring transiently 

 and rarely in Microderoceras Birchii, but becoming characteristic of 

 the adult in Microceras biferum, and the prominent peculiarity of the 

 remaining forms of this genus. 



That this is not an artificial arrangement may be seen by consult- 

 ing the geological succession of the groups. Microderoceras Birchii 

 is found in the " Tuberculatusbett " of Oppel. Microceras biferum 

 and Microceras laticosta 1 in the " Oxynotusbett," the latter, how- 

 ever, lasting into the middle Lias. In this formation it overlaps 

 Androgynoceras, which appears in the " Jamesonibett," followed, and 

 perhaps associated, with Liparoceras Henleyi and Liparoceras Becliei. 



Ophioceras raricostatum, with its keel and septa, development and 

 form, allying it closely with the Arietidae, and indicating that its true 

 position is at the head of a series of this family, occupied geologically 

 an earlier position in the " Raricostatusbett " of the lower Lias, than 

 the two species which resemble it in the middle Lias. These are, 

 undoubtedly, part of the Amm. capricomus of Oppel, and are there- 

 fore found in the " Davoibett " of that formation. 



The planicostan abdomen which occurs occasionally in the young 

 of raricostatum before the keel appears, leads to the conclusion, if we 

 credit the hypothesis of evolution, that Microceras crescens and Mi- 

 croceras arcigerens derived their peculiarities from the same source, 

 and are either directly or indirectly the descendants of this or some 

 other common ancestor. I am disposed to credit the latter supposi- 

 tion. The septa examined were those of young specimens, and in 

 the case of the last named it will be noticed that the resemblance is 

 remarkable in the external features of the shell as well as the septa. 

 No one, however, I am confident, without having subjected them to 

 the closest scrutiny would suspect that they could be separated from 

 Microceras laticosta, with which they are also associated geologically. 

 Again, this species is genetically connected with Microceras biferum 

 on the one side and with Androgynoceras hybridum on the other. 

 According to Quenstedt, the former species is hardly separable in 

 some of its varieties from Ophioceras raricostatum, but if the septa are 

 examined closely they are found to differ, and the young are different. 



1 Microceras laticosta here includes also the Amm. capricomus of Oppel and the 

 two species alluded to in the note above, as M. maculatum and 31. sinuosum. 



