Hyatt.] 24 [October 5, 



The series which we are now considering, has three other closely- 

 allied species in the middle Lias. 



The first is Microceras laticosta, 1 whose young are precisely similar 

 in all respects to Microceras biferum, though the adults differ consider- 

 ably, the planicostan abdomen being brought out more distinctly in the 

 adult stage than in biferum. Associated with this species is Microce- 

 ras crescens, whose septa in the young have the same characteristic 

 outline and proportions as in the adult of Ophioceras raricostatum, 

 though the whole form and external features identify it with Micro- 

 ceras laticosta. Then there is Microceras arcigerens, whose septa in 

 the young are like those of the compressed form of raricostatum at 

 an earlier age, just before the minor lobes and cells attain a decided 

 prominence. 2 The whorls in this species are flattened dorso-abdom- 

 inally. The dorsum is broader than the abdomen, and this, together 

 with the flattened aspect of the whorls and the early development 

 of the closely set pilse, gives an umbilicus closely simulating that of 

 raricostatum. 



The genus Androgynoceras returns to the peculiar pilae and tuber- 

 cles of Microderoceras Birchii in the adult, though retaining the adult 

 characteristic of Microceras until a late stage of its growth. This 

 is especially remarkable in. Androgynoceras hybridum (D'Orb.), but 

 becomes confined to an earlier stage in Androgynoceras appressum. 



The next genus of this same genetic series exhibits in Liparoceras 

 indecisum the planicostan abdomen not later than the fourth whorl. 

 In Liparoceras Henleyi this is apparent at an earlier stage only, and 

 in Liparoceras Becliei it is absent altogether. 



The same mode of growth is here returned to, which was first ob- 

 served in Microderoceras Bircliii] namely, a smooth, round whorl, suc- 

 ceeded immediately by two lines of tubercles or spines, erected upon 

 pilsB which do not cross the abdomen, except as fine, distinct linear 

 ridges. The difference between the two species, in other respects, is 

 very great, sufficient, in fact, to constitute very distinct genera. It 

 will be observed that we have here a closed series, one in which the 



x la. the Bulletin of the Museum of Comparative Zoology, No. 5, this species ap- 

 pears under the names of Microceras sinuosum and Microceras maculatum, two 

 species which I now regard as the compressed and gibbous forms of Mic. laticosta. 



2 This compressed variety is the one figured by Sowerby, and can only be doubt- 

 fully referred to the same species as O. raricostatus, which is much flatter on the 

 abdomen, and altogether different in form as well as smaller, though precisely sim- 

 ilar in the septa. 



