﻿SONNININiE. 



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SoNNTNiNyE {continued). 

 Genus — Sonninia, Bayle. 

 (Type — Sonninia pbopinquans, Bayle.) 



— Habpoceeas (pars) auctorum. 



— Hammatocebas (pars) auctorum. 



1878. Waagenia, Bayle. Explic. carte geol. France ; Explan. of pi. lxxxiv. (Nod 



Neuinayr.) 



1879. Sonninia, Bayle. Bull. Soc. Geol. France, vol. vii, ser. 3, p. 92. 



1885. Gruppe des Hammatocebas Sowebbyi, Haug. Beitr. Monogr. Harpo- 



ceras ; Neues Jahrbuch fiir Mineral., &c, 

 Beil.-Bd. iii, p. 654. 

 1885. Gruppe des Hammatocebas Ogeeieni, Haug. Ibid., p. 657 (pars). 

 1889. Sonninia, S. Buchman. Descent of Sonninia, &c, Quart. Journ. Geol. 



Soc, vol. xlv. 



It is impossible to give any short and exact definition of this comprehensive 

 genus. The biologically-earliest species belong to the spinous stage from early 

 infancy (PI. XLIX). In some cases the spinous stage persists to maturity, but 

 generally it retrogrades to a costate stage in adolescence, or even in infancy. 

 In biologically-later species the costate stage retrogrades to a striate in 

 maturity, in adolescence, or even in infancy, and the spinous stage may 

 be omitted from the ontogeny. 1 Roughly speaking inclusion and compression 

 of whorls correlate with the amount of ornament — the most ornate species 

 being the most evolute, and having almost circular whorls ; but the exactitude 

 of the correlation varies in the different branches into which the genus 

 splits up, and in one branch it is altogether falsified on account of a 

 remarkable renewal of progressive development after a period of retrogression. 

 The hollow carina is not always present, but it is the rule. In the extremely retro- 

 gressive forms of the genus, however, not only is the hollow carina absent, but 

 the keel is altogether lost, and the ventral area becomes quite rounded in maturity. 



The suture-line is an important feature. The lobes are generally narrow- 

 stemmed, in some species exceedingly so ; the superior lateral lobe has a 

 peculiar cruciform arrangement of its chief terminal branches, which are long and 

 important ; 2 the first auxiliary lobe often has a slight twist towards the inferior 

 lateral ; the inner end of the suture-line is not, practically speaking, dependent or 

 retracted. In general the complexity of the suture-line increases in proportion 



1 To favour direct development, illustrating the law of the partial or modifying action of earlier 

 inheritance. 



2 In some of the broad-whorled forms the symmetry is destroyed by an excessive growth of the 

 outer lobule. 



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