TMETOCERAS SCISSUM. 273 
figure of " Am. scissus" (Bassin Rhone, iv, pi. Ivii, fiigs. 1. 2), because this form 
has certain bifurcate ribs ; but I cannot give an opinion as to its specific value. 
In this country, however, so far as I know, only two species, Tmetoc. scissnm 
(Benecke) and Tmetoc. HoUandse (S. Buckman), have been found. 
The range of this genus is from the Jurense-zone^ to the Jlurchisonse-zone. 
Tmetoceeas SCISSUM {Benecke). Plate XLVIII, figs. 1 — 10. 
1768. Ammonite, Knorr. Monumens des Catastrophes, pi. A v, fig. 3. 
1868. Ammonites scisstrs, SenecTce. TJeber Trias und Jura, pi. vi, fig. 4. 
1874. — Eegi.eti {v. TJiiolliere), Dumortier. Bassin du Rhone, iv, pi. 
xxxi, figs. 8, 9. 
1878. CosMOCEEAS Kegleti, Gotische. Jurass. Verstein. Cordillere ; Palseout. 
Suppl. iii, Lief ii. Heft 2, pi. ii, fig. 3. 
1883. — Holland.!;, S. Buchman. New species of Ammonites ; Proe. 
Dorset. Nat. Hist. Club, vol. iv, 
pi. i, fig. 2 (non pi. ii, fig. 2). 
1886. SiMOCEEAS SCISSUM, Vacel'. Ool. Cap san Vigilio; Abh. k.-k. geol. 
Eeichsanstalt, Bd. xii, pi. xvi, figs. 15 — 17, 
Discoidal, ventral area sulcate. Whorls circular to oval in section, ornamented 
with very prominent, annular or sub-annular ribs, which are interrupted on the 
middle of the ventral area, where they produce a blunt obscure tubercle, and 
then die away with a slight forward twist. The ventral area is not defined ; in 
some specimens it is sulcate in the interspaces between the ribs, in others only 
the ribs are broken. Inner margin not defined. Inclusion one-fourth to one- 
third. Umbilicus open, scored by numerous ribs. Suture-line simple ; superior 
lateral lobe about as long as siphonal, inferior lateral and auxiliary lobe not 
dependent. 
In 1883, being iniacquainted with Benecke's figure of Am. scissus, I described 
and figured with the name Cosmoceras Hollands, the specimens depicted in figs. 
4, 5, of PI. XLVIII. Although our English examples do not show the periodic 
constrictions, or wider interspaces, observable in Benecke's figure, I do not think 
they deserve a separate specific name. I imagine that our examples represent a 
dwarf form of the Continental species, and that the constrictions may be absent 
owing CO their small size. 
^ Dumortier's A?n. Ueglei/i (Tmetoc. scissum) (' Bassin Rhone,' iv, p. 120) is given as from the 
Bifrons-zone, which is equal to the strata from the Commune- to the Jurense-zone inclusive. 
35 
