62 GENESIS OF THE ARIETID. 
of the pil and of the channels, but often left the keel more prominent. The 
whorl acquired the true flat-sided trigonal form, but never became rounded, so 
far as yet observed. 
LEVIS STOCK. 
Arnioceran Series. 
This series begins, when zodlogically considered, with Arn. miserabile, a form 
very commonly in collections named Amm. planorbis or psilonotus, on account of its 
close external resemblance to that species. It has, however, distinct sutures, and 
acquires by growth a subacute abdomen. During the younger stages, while it is 
still round on the abdomen, or in varieties with broad abdomens, it is, with the 
exception of its smaller size, a close reproduction of the adult of Psil. planorbe, 
var. deve. This grades into Arn. miserabile, var. cunciforme, which has a more 
acute abdomen and curved and more perfect pile acquired at an earlier age, 
and from this without a break the series passes into Arn. obtusiforme.* 
Starting again from variety acutidorsale of miserabile, we can follow another 
line of affinities. There are some forms of this variety which acquire in the 
adult a keel with faint but abruptly terminating folds or pile, and these lead into 
a variety of Arn. semicostatum.” ‘This species has many varieties, which grade 
from an immature planorbis-like form® to those which are prominently keeled 
and pilated even at a comparatively early age,’ and also into varieties which bave 
deep channels on the abdomen.’ These last are inseparable from Arn. tardecres- 
cens,’ and when they have numerous pilz they are inseparable from Arn. ceras.” 
There are also varieties of Arn. semicostatum which fade into Hartmanni, and 
this in turn grades into the still more compressed Bodleyi?? From Arn. Hart- 
manni, also, we can pass into another compressed form, the true Amu. Saleuries 
of Quenstedt.¥ 
Returning again to senucostalum, we find that one of its varieties is distin- 
guished for remarkably raricostatus-like pila and alow keel. This when fol- 
lowed out leads to Arn. kridioides.* This species in some of its varieties so closely 
resembles Cal. raricostatum, that for a time it was thought to indicate the direct 
descent of that species from semicostatum. 
There are also some forms, usually identified as Anum. kridion in Germany, 
which have remarkably broad whorls in the adult, and approximate to the true 
kridion. These, however, never possess the tubercles of the true sridion, and 
also have young which prolong the smooth stage and otherwise resemble the 
young of the stouter forms of semcostatum. 
2 Pleue tig. 4,0; Summ, Pl. xi io, 2, 
* The apertures are also similar as figured by Quenstedt, Amm. Schwab. Jura, pl. xiii. fig. 27, by Du- 
mortier, 
ohn fig, 4 Pl. ii. fig. 8,9; Summ. Pl. xii. fig. 3. 
* Pl. iu. fig, 10; Summ. Pl. xii. fig. 14. os iy 155 0, 
"Plea. figs 11; Paranoia; 
® Pl. il. fig. 19; Summ. Pl. xii. fig. 6, EL, Wet 20, 20 a, 
aEl uote. 17; Summ, Pl xii. figs 5. ~ El ue fig. 25; Sutin, Pl, xii. fies 7. 
18 Pl. Me tig, 20-27, “ Pl, i. fig. 28; Summ. Pl. xii. fig. 8. 
