ARCESTES. 239 



of the Liassic Ammonites receiving scant notice, and his genera 

 not even enumerated because " they do not agree with natural 

 groups."f 



The classification appears to be conservative in spirit, and cal- 

 culated to subserve usefully the needed grouping of the numer- 

 ous species which overburthen the original genus. The synonymy 

 is unfortunately rendered inextricable by the great difference 

 of opinion as to valid characters entertained by several recent 

 systematists. who appear to have each done their best to increase 

 the prevalent confusion, by forming groups which will not 

 coalesce entirely with those of their contemporaries or prede- 

 cessors. 



The following" is an epitome of Nenmayr's arrangement : 



Family I. AKCESTIDJv 



Shell smooth or with transverse folds, ribs or stri;e ; wrinkled 

 layer present in the geologically older forms, consisting mostly 

 of linear, interrupted stria 1 , seldom (only in Sageceras) granular; 

 impressions of the mantle attachment, in the triassic forms, with- 

 out or with a but slightly contracted opening always visible on 

 the body-chamber. Anaptychus apparently horny in Arcestes. 

 certainly present in J inolt/iett*. doubtfully so in the, other forms. 



(jicmis ARCESTES, Suess. (<>. pa i-le). 



Shell, as a rule, smooth, sculptureless, seldom with longitudinal 

 striae (Tornati) ; body-chamber long, taking up one to one and 

 a-half whorls. Whorls strongly involute. Aperture usually con- 

 tracted by the border being reflected inwards or by internal ridges. 

 Lobes strongly incised (laciniated), so that the saddles merely 

 consist of a slender stem with numerous approximated horizon- 

 tal branches, which in turn are divided into smaller branchlets. 



Many forms have internal nuclei with an open umbilicus, and 

 a terminal whorl with a callous closed umbilicus. 



130 species Trias; one species Permian. 

 ARCESTES TORNATTJS, JJronn. T. 108, figs. 631, 632. 



f Prof. Hyatt very properly protests against ignoring prior generic 

 names on account of a difference of opinion as to the extent of the 

 groups and the relative importance of the characters given. See Bost. 

 Soc. Proc., xviii, 360, 1876. 



