66 AMMONITID^. 



Arcestes, Suess, 1865. 



Distr. — 130 sp., Trias ; several sp. Permian,. and Carboniferous 

 of India ; 1 sp. Trias, IST. Caledonia. Arceates tornatus, Bronn 

 (xxxviii, 20, 21). 



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

 striae (Tornati); body-chamber long, takii:g up one to one and 

 one-half whorls. Whorls strongl}^ 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 hori- 

 zontal 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. 



ARCESTES (restricted). Last whorl of the spire more or less 

 modified in form ; umbilicus closed by a callous deposit. 



SPHiNGiTES, Mojs., 1879. Surface of the last whorl sillonated ; 

 umbilicus open. 



CLADisciTES, Mojs., 18*79. Section of the whorls subquadran- 

 gular, their form not modified in the adults. The character of 

 the suture recalls the Pinacocerae ; second lateral lobe deep, 

 saddles slender, much divided. 



JOANNiTES, Mojs., 18'79. Sutural lines like those of Cladiscites, 

 but the other charactei-s those of Arcestes. The sutural line is 

 arcuated, lobes and saddles partly divided in pairs, much toothed. 



DiDYMiTES, Mojs., 1873. External form and length of body- 

 chamber same as in Arcestes ; shell with sharp lines of growth 

 and plicate wrinkles throughout the whole length of the body- 

 chamber to the aperture ; on the inner convex surface of the 

 shell there is a median furrow ; the last whorl is constricted near 

 the aperture. 



The sutural lines of the septa are formed of few-toothed 

 saddle pairs, which often alternate with single saddles. These 

 saddle pairs, as is shown by projection of the spiral, correspond 

 each to two saddles in the other genera of Ammonites. 



Didymites contains but a few Triassic forms. Didymites 

 anguatilobatus, Hauer (xxxix, 32, 33). 



Cyclolobus, Waagen, 1879. 



Distr. — C. OM/iamt, Waagen (xxxvi, 93, 94). Palaeozoic ; India. 



^hell spiral, smooth, with a small deep umbilicus ; whorls 

 numerous, somewhat compressed, with a broadly rounded exter- 

 nal side, deeply embracing each other so as to envelop the 

 preceding whorl, entirely covered on the sides with not very 

 numerous contractions of the shell (varices), indicating very 

 likely th'e remnants of old apertural margins. The form of these 

 varices is somewhat falciform, with a strong bend towards the 

 front in the middle of the sides of the whorl, and bein^- bent 



