348 INFERIOR OOLITE AMMONITES. 
spinous stage, so that there is a great contrast between the ornamentation of the 
inner and outer whorls. 
The various species of the crassa-stock may be separated from one another 
thus : 
S. crassifornmis, spinous to bullicostate.' 
S. crassa, centrally spinous, but mostly costate. A very thick form, 
especially considering the comparatively small umbilicus.” 
S. crassinuda, centrally spinous, adult smooth. 
S. nuda, almost completely smooth. 
These species illustrate various phases of retrogression. Parallel herewith 
runs the other series, in which the later spinous stage has its spines at first 
rather distant, but as the series develops they close up—the effect presumably of 
accelerated development,—so that the infantile stage of the later forms has a 
remarkable likeness to adolescent crassiformis. In point of development the first 
species may be placed between crassiformis and crassa. These species are— 
S. crassibullata, spinous, bullicostate to costate. 
S. crassicostata, the same, but losing the ribs very early. 
S. diversa, with numerous spines, then subcostate. 
S. levigata, like one form of crassinuda, but with a very deficient 
spinous stage. For the most part nearly smooth. 
The genealogical position of the crassa-series cannot be exactly determined— 
on this point see the following article on crassiformis; but, so far as its general 
appearance is concerned, crassiformis takes a place between acanthodes and 
irregularis. 
SONNINIA cRASsIFoRMIS, S. Buckman. Plate LX XIX, figs. 1—6. 
Discoidal, compressed, hollow-carinate. Whorls, in section, elliptical, 
ornamented first with strong, regularly-placed spines set on slightly reclining 
obscure ribs, which part into two or three beyond the spines; later with large 
single, direct, almost upright,’ ventrally-inclined coste, which are elevated in the 
middle to blunt elongate knobs (bullicostate). Ventral area arched, divided 
by a strong, rather thick hollow carina. Inner margin only in adult; then 
subconvex, upper part rounded off. Inclusion in youth up to the spines; in adult 
1 See page 353, footnote 1. 
2 The general law of retrogression is that the whorls become more compressed as the umbilicus 
becomes smaller, so that in proportion to the size of its umbilicus this species is very thick-whorled. 
3 The cost become more upright with age, and are quite upright at the end of whorl. 
