1870.] 23 [Hyatt. 



notum in its septa, and the latter also in its external characteristics. 

 None of the lower forms, however, display, so far as observed, the 

 planicostan stage, though they occur earlier than the species which 

 do exhibit this peculiarity. The planicostan abdomen, therefore, 

 must either be a new characteristic suddenly interpolated in the 

 growth of some individuals, or a reversion to certain ancestral char- 

 acteristics which have been discontinued for a time in the lower 

 members of the family. 



The lower forms of the Arietidas, Caloceras torus and such species 

 as Amm. nodotianum, with which this species is closely allied, have 

 septa that, are similar to those of certain Triassic species, such as 

 Amm. Brunneri 1 and Amm. Batteni Strachey, which also resemble 

 Psiloceras psilonotum in their septa and forms. 



The affinity, therefore, is doubly proved through the latter species, 

 which is a contemporaneous form, and by direct comparison. Besides 

 these there are other species, such as Amm. Iceoidorsatus Hauer, and 

 Clydonites quadrangulus Hauer, which show us that the planicostan 

 abdomen is by no means a new feature. Thus, though we cannot 

 assert that the Arietidas are directly traceable to species in the Trias 

 having the planicostan abdomen, we can say that the family on its 

 lower borders have affinities with Triassic species, and that the plani- 

 costan abdomen is found in the Trias. It is probable, therefore, that 

 the same modification, when it occurs in the higher Arietidae, after a 

 certain interval of time is a reversionary feature. 



The young of Coroniceras nodosum, Amm. Sauzeanus of D'Orbigny, 

 is succeeded in the next bed, the " Tuberculatusbett," by a new form, 

 Microderoceras Birchii, whose young are entirely distinct in their 

 mode of development from any of the Arietidas. They are at first 

 very cylindrical and smooth, then two rows of tubercles are intro- 

 duced; and sometimes, though rarely, a specimen occurs in which 

 the planicostan abdomen is presented. The septa develop to a 

 more complicated outline in a shorter time than any of the species 

 which follow in the same series or any species among the Arietidae. 



Very similar to this in its adult ornaments and septa is Microceras 

 blferum ; in fact, I was disposed to think them members of the same 

 genus, until I became aware that a representative species, " Microd. 

 Hebertii" existed in the middle Lias. This establishes a distinct 

 series for Birchii, and makes it necessary to employ a different name. 



1 Haidinger's Abband., Bb. 3, p. 23, pi. v, figs. 7-9. Mem. Geol. Survey of India, 

 Stol., vol. v, pi. i, p. 59; pi. v, figs. 2, 3. 



