66 GENESIS OF THE ARIETIDA. 
due to the stout whorls of the younger stages, and cannot be relied upon as at 
all conclusive. The stout helmet-shaped whorl is a larval characteristic, derived 
from the primitive ancestral Goniatitic form, Anarcestes. It is found in all the 
Ammonitine at an early stage of growth, and may be retained in radical forms 
until a late stage; and in this species it sensibly influenced the shape of adults. 
The adults of many specimens of Agas. /evigatum' are closely parallel with 
Psil. planorbe. Dwarted specimens sometimes have the form and smooth aspect 
of planorbe, and even the apertures are similar, and Agas. levigatum, therefore, 
must have been a direct descendant of Psil. planorbe.2 In var. d of devigatum the 
depressed helmet-shaped whorl is exchanged in course of growth for the com- 
pressed helmet-shaped, just as in planorbe. The young,’ unlike the adult of 
planorbe, have short living chambers,‘ and the septa are quite distinct, and im 
most specimens there is a raised siphonal ridge along the abdomen, though the 
keel is not well developed, nor are the channels present. The misnaming of 
varieties of levigatum as Amm. planorbe is also common in European collections. 
The same peculiarities are present also in Agas. striaries. In some varieties 
of this species there are perhaps still closer approximations to the general form 
and aspect of the smooth forms of Psil. planorbe. 
Agas. Scipionianum ® has two marked varieties, — one less involute, with spines 
in the adult,’ and one more compressed, with smaller spines.’ 
Agas. nodosaries is apparently a compressed form, very similar to Scipiomanum. 
Agas. Scipionis is a distinct species, having smooth and more involute whorls.” 
The Coroniceran proportions and aspect of the sutures in Se/piomanum are well 
marked, and would have led to the association of this species with that genus 
if there had not also been similar sutures in Ast¢. obfuswm, showing that these pro- 
portions are progressive characteristics of independent origin in each series. 
The completeness of the gradations from the adults of Agas. striaries to the young 
of this species also forbids this conclusion. 
Asteroceran Series. 
This series has two subseries. 
First Subseries. —The more advanced varieties of Agas. lavigatum" have diver- 
gent-sided whorls and fold-like pile, and a form™ similar to the tuberculated 
young of Ast. obtuswum,” and still more like the untuberculated young of this 
species.” In the accelerated development of the tuberculated variety of obtusum," 
iPr, vate 9, 12. 
2 Compare the young of the last named, fig. 4, pl. i., with fig. 10, pl. viii. 
8 Pl. viii. fig. 13. 
4 Short living chambers are found in the young of Psil. planorbe, and therefore this characteristic is 
really a confirmation of the assumed direct descent of Agas. levigatum from that species. Agas. levigatum is 
an arrested development of Psil. planorbe, in so far as the living chambers and its small size are concerned. 
5 Pl. ix. fig. 14, 15; Summ. PI. xiii. fig. 6. 6 Pl. x. fig. 11-13; Summ. Pl. xiii. fig. 7. 
PDL Seriiek belo, 8 Pl. x. fig. 13, and pl. vii. fig. 15. 
9 Summ. Pl. xiii. fig. 8. 10 P). viii. fig. 11. 
1 Pi, viii. fig. 14. 2 Pl. viii. fig. 8. 
18 Embryology of Cephalopods, pl. ti. fig. 11. 14 Pl, viii. fig. 4. 
