12Q GENESIS OF THE AELETIDvE. 



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 pilse, 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, Wah., 

 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, Hyatt. 



Amm. subangulare, Oppel, Paleontolog. Mittheill., p. 130. 



We have referred the species to this genus entirely upon the information 

 derived from notes made before Waehneroceras 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 Wrehneroceras. 



One of the types of Amm. subangularis, Oppel, from Kalthenthal, in the collec- 

 tion at Munich, has a form similar to that of Psil. 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 pilae, and the form and smooth abdomen of 

 planorbe (not channelled at all) until over 26 mm. in diameter; then the piloe 

 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 angulata, and one of 

 them has a very peculiar old whorl, and may be a caloceran form. 1 



Waehneroceras tenerum, Htatt. 



JEgoc. tenerum, Xkvm , Unterst. Lias, Abh. k. k. geol. Reichsans., VII. pi. iii. fig. 4, 5. 

 Psil. t< hi rum, Wah., Unt. Lias, Mojsis. et Xeum., Beitr., IV. 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 Week, 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 ScMot. angulata. 



Species of tliis series have been figured by Wanner in Volume III. of his 

 " Unteren Lias" as follows : Week. Pattar, Plate XXL, and Rahana, Plate XXIII. ; 



1 I'roc. Bost. Soc. Xat. Hist., XVII , 1S74, p. IS. 



