18 GENESIS OF THE ARIETID.E. 



has not been ascertained in any one form, so far as we know. The microsi- 

 phonula occurred, as might have been expected, earlier than the true goniatitic 

 stages, or goniatitinula, in those species which had the nautiloidean stage with 

 ventral saddle also prolonged into the second septum, as in the Asellati figured 

 by Sandberger, and Gon. cdratus figured by Branco. The goniatitinula became 

 distinguishable when the first ventral lobe appeared. This was undivided, as 

 in the lower Anarcestes and in the Magnosellaridse among Goniatitinse. This 

 stage is prolonged through one or more septa in the higher Goniatitinse, and 

 also in the Lytoceratinae and Ammonitinse, and the whorl also at this time 

 strikes one as similar to Anarcestes, or depressed semilunar in section, as stated 

 above, and in these the goniatitinula is completed. 



The duration of the nsepionic period can in a general way be described 

 as coincident in extent with the duration of the smooth shell, which is always 

 found at the centre of the umbilicus, however much the shell may be subse- 

 quently ribbed and ornamented. This period would of course include many 

 more transformations than the goniatitinula, especially among the higher and 

 later occurring species of the Mesozoic. 



Haeckel designated all of the progressive stages which succeeded the true 

 ovarian stages and included the nrepionic and nealogic stages, and their 

 structural relations, under the term Metamorphology. 1 This term is, however, 

 somewhat indefinite and artificial when limited in this way, since the ovarian 

 stages are necessarily of very different duration in distinct groups, and cannot 

 be considered as the natural limit of the embryologic period. We should, as 

 above stated, be disposed to think that some such limit as here proposed would 

 be nearer to the true one, namely, to consider the typembryos as the last of 

 the true embryologic stages. This nomenclature would enable an author to 

 give an approximate idea of the stage at which the metamorphologic stages 

 began in any type. Thus, they would have begun in Nautiloids with the asi- 

 phonula, and in the absence of this among Ammonoids with the caecosiphonula. 

 In the absence of this last, if it is absent among the lower Sepioidea, the meta- 

 morphologic stages, according to the same rule, would begin with the first stage 

 immediately succeeding the protoconchial stage. Whenever this last is absent, 

 as it certainly is among the highest of the Sepioidea having meroblastic ova, 

 then its equivalent stage, which represents what is left of the veliger, should 

 be taken as the last of the embryologic stages. 



As has been noted above, the nsepionic period is always smooth, and is 

 visible at the centre of the umbilicus in most discoidal shells, and the demar- 

 cation is therefore visible between this and the nealogic period ; but, as can be 

 observed on most specimens, an attempt to separate the characters of the latter 

 from the characters of adults is attended by greater difficulties. It is, however, 

 essential to distinguish the category of ephebolic or adult characters from the 

 nealogic, because in each form of any series there are usually found certain novel 

 characters, which appear for the first time in that particular series. These make 

 their first appearance almost invariably during the ephebolic period. 

 1 Morph d. Organismen, II p. 22. 



