66 GENESIS OF THE ARIETID^E. 



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 

 Ammonitinse 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. Icevigatum l are closely parallel with 

 Psil. planorbe. Dwarfed specimens sometimes have the form and smooth aspect 

 of planorbe, and even the apertures are similar, and Agas. Icevigatum, therefore, 

 must have been a direct descendant of Psil. planorbe. 2 In var. d of Icevigatum the 

 depressed helmet-shaped whorl is exchanged in course of growth for the com- 

 pressed helmet- shaped, just as in planorbe. The young, 3 unlike the adult of 

 planorbe, have short living chambers, 4 and the septa are quite distinct, and in 

 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 Icevigatum as Amm. planorbe is also common in European collections. 



The same peculiarities are present also in Agas. striaries. 5 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 6 has two marked varieties, — one less involute, with spines 

 in the adult, 7 and one more compressed, with smaller spines. 8 



Agas. nodosaries is apparently a compressed form, very similar to Scipionianum,. 



Agas. Seipionis is a distinct species, having smooth and more involute whorls. 9 



The Coroniceran proportions and aspect of the sutures in Scipionianum 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. obtusum, 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. Icevigatum 10 have diver- 

 gent-sided whorls and fold-like pilos, and a form 11 similar to the tuberculated 

 young of Ast. obtusum, 12 and still more like the untuberculated young of this 

 species. 13 In the accelerated development of the tuberculated variety of obtusum, 1 * 



1 PI. viii. fig. 9, 12. 



2 Compare the young of the last named, fig. 4, pi. i., with fig. 10, pi. viii. 



8 PI. viii. fig. 13. 



4 Short living chambers are found in the young of Psil. planorbe, ami therefore this characteristic is 

 really a confirmation of the assumed direct descent, of Agas. laevigatum from that, species. Agas. Icevigatum is 

 an arrested development, of Psil. planorbe, in so far as the living chambers and its small size are concerned. 



6 PI ix. tig. It i:,; Summ. l'l.xiii. fig. 6. « PI. x . fi„ n-13; Summ. PI. xiii. fig. 7. 



' P1 - x - & S- > I- 12. s P1 x fig 13 and p , vii fig 15 _ 



9 s """ 1 ' PI' xiii. fig. 8. 10 P1 . viii fig u 

 11 Vl «ii. fig. 11- >'•> PI. viii. fig. 8. 

 13 Embryology of Cephalopods, pi. ii. fig. 11. '« PI. viii, flo-. 1. 



