126 GENESIS OF THE ARIETIDA. 
become wider apart, the abdomen narrower, and the whorl consequently much 
compressed and subacute. No proper quadragonal whorl is formed during the 
growth, and therefore the senile outline of a section of the whorl is not trigonal, 
as in the senile stages of shells of other branches having a flattened abdomen 
and a keel in the ephebolic stages, All the species have true pile, though these 
are not prominent, and the earlier nealogic stages resemble adult specimens of 
the second subseries of Psiloceras, in which the folds are well developed and 
cross the abdomen. We cannot distinguish either this genus or Schlotheimia, 
or Caloceras, from Psiloceras by means of the sutures. Psil. sublaqueum, Wiih., 
and other species of Psiloceras having plications which cross the abdomen until 
a late stage of growth, are not distinguishable until they are nearly full grown 
from some discoidal forms of this series. 
Waehneroceras subangulare, Hyarr. 
Amm. subangulare, OrprEL, Paleontolog, Mittheill., p. 130. 
We have referred the species to this genus entirely upon the information 
derived from notes made before Waeehneroceras was separated. It will be seen, 
however, that no species of Schlotheimia has young which remain similar to 
Psiloceras for such a prolonged stage as in Weehneroceras, 
One of the types of Amm. subangularis, Oppel, from Kalthenthal, in the collec- 
tion at Munich, has a form similar to that of Ps. planorbe, and pilations which 
cross the abdomen. The young is also a pure planorbe until over 14 mm. in 
diameter. Another specimen from Filder, which we have referred also to this 
species, has curved and close-set pila, and the form and smooth abdomen of 
planorbe (not channelled at all) until over 26 mm. in diameter; then the pilee 
begin to cross the abdomen. ‘This last specimen was named Amm. planorbis by 
Oppel. There are also specimens from Hammerkhar, formerly referred by us to 
subangulare, which may be distinct. They certainly possess characters which were 
noted by us as intermediate between this form and true angulala, and one of 
them has a very peculiar old whorl, and may be a caloceran form.! 
Waehneroceras tenerum, Hyartr. 
4igoc. tenerum, Neum., Unterst. Lias, Abh. k. k. geol. Reichsans., VII. pl. iii. fig. 4, 5. 
Psil. tenerum, Wiu., Unt. Lias, Mojsis. et Neum., Beitr., 1V. p. 198. 
This form, described by Neumayr as occurring in the Northeastern Alps and 
also in Central Europe, at first seemed to us identical with Wah. subangulare. 
Neumayr remarks that, though the young are so similar, the adults are separable, 
and we have upon his authority held it to be distinct. He also looks upon. this 
species as very closely allied to Psiloceras, and to be a transition form from the 
latter to Schlot. angulata. 
Species of this series have been figured by Wihner in Volume III. of his 
“Unteren Lias” as follows: Woh. Paltar, Plate XXL, and Rahana, Plate XXII; 
' Proc. Bost. Soc. Nat. Hist., XVII., 1874, p. 18. 
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