50 GENESIS OF THE AKIETID.E. 



much obscured by morphological equivalents, and in such series the closest study 

 of the structural gradations becomes the only sure guide. 



As a rule, a series also runs through a gamut from the discoidal forms to the 

 involute, but not always, because there are series like Caloceras having no involute 

 forms, and no one species of Vermiceras or Arnioceras is more involute than another. 

 Nevertheless there is a decided developmentof the quadragonal whorl in Vermiceras, 

 which, as shown in the series of species in Coroniceras and Asteroceras, and during 

 the development of the individual in all normal forms of the Arietida;, is usually 

 an intermediate stage to the future genesis of compressed and involute shells. 



In such a system, also, certain radical forms which do not show the usual 

 morphogenetic cycle may occur, as was the case with Psiloceras before the more 

 involute forms of that genus 1 were discovered in the Mediterranean province. 

 These may have a closely allied and inseparable series of varieties, 2 which 

 cannot be distributed into the different genera arising from them. In such 

 cases, the radical may be considered as an undeveloped series, and separated 

 as a distinct genus, though it consist of but one species with well marked varieties. 



A species is a definite step, or gradation in the morphogenetic cycle of the 

 genus, and is distinguished by its form, amount of involution, sutural and other 

 adult and senile characters, and the more or less accelerated development of 

 the nealogic stages. In the descriptions, it will be noticed that the ephebolic 

 characters of the ancestral form, though it may be a closely allied species, are 

 nevertheless often accelerated in the nealogic stages, and the ephebolic stages 

 then acquire some peculiar distinctive differentials. The aberrant pathological 

 forms, and dwarfs of the same species, may often have more accelerated devel- 

 opment than the normal forms, and sometimes simulate distinct species. These, 

 as well as the normal varieties of species, have connections with other species 

 which can only be properly estimated with sufficient materials and accurate 

 study. After having secured the genealogy of a series, the species can be deter- 

 mined and separated, but until this is done, the work does not rest upon a secure 

 basis. The possession of a keel, or channels, or a line of tubercles, or increased 

 involution in the adult whorls, may distinguish one species from another in the 

 same series ; but the same differences may make the shell appear to be identical 

 with a species occurring in another genus, and thus confuse the classification 

 unless the genesis of the characteristics has been traced. 



The order adopted for illustrating the series in the Summary Plates is the 

 result of following out genetic lines, and therefore presents forms in their ap- 

 proximately natural relations, though necessarily having no reference to chronol- 

 ogy. The species are connected by lines indicating their natural affinities, and 

 show the relations of the series; but the title, Summary Plates, fully explains 

 the necessarily abbreviated and more or less artificial nature of the arrange- 

 ment. Comparison with Genealogical Table V. will serve to correct any erroneous 

 impressions which might arise from the study of these plates, in so far as the 

 species of Western Europe are concerned. Those from other localities, also 

 figured in the Summary Plates, will be found by reference to the descriptions. 



1 Summ. PI. si. fig. 11-13. "- Suram. PI. xi. fig. 1, 2 ; PI. xii. fig. 1. 



