402 CRETACEOUS GASTROPODA 
to the Prraurpettipz. This last specific name hasas yet to be reserved for species, 
like I. Alabamiensis (Contrib. p. 186, pl. 6, fig. 201), which is a shell in form 
resembling an Ancilla, being anteriorly truncated and possessing on the posterior por- 
tion of the inner lip an oblique fold, IZ. cwrta and Leai, Con. (Am. Journ. Conch. I, 
pp- 148 and 261) being very similar forms. Lea (loc. cit., p. 185), says that he 
obtained by a vessel coming from Calcutta a small recent shell which, he thinks, 
belongs to the same genus, distinctly pointing to a sinus at the base and a simple 
columellar fold, but in form resembling a Delania, for which reason he calls it 
Monopt. melanoides. 
7a. Leucotina, Adams, 1860 (Ann. mag. nat. hist. V, p. 406), with the recent 
type species LZ. Niphonensis, only differs from Myonia by a somewhat more ventri- 
cose form of the last whorl. A. Adams calls the shell intermediate between Iyonia 
and Acteon. I do not know any cretaceous species which distinctly resemble the 
typical one of Myonia, but D’Orbigny figures an Acteon marginata (Pal. frane. 
eret. IT, pl. 167, fig. 8) which could be referred to Leucotina, having the fold rather 
posterior and the aperture anteriorly produced. 
8. Acteon, Montfort, 1810 (Lornatella, Lamarck, 1812). There is only a 
slight gradual distinction between the genera Act@on and Wyonia ; in the former the 
spire being always shorter than the last whorl, while in the latter the spire is of 
eveater height, and in Lewcotina of about the same as the last volution. 
8a. Solidula, Fischer, 1807 (Buccinulus, Plancus, apud H. and A. Adams, 
Gen., II, p. 5) is distinguished by two (and occasionally more) folds on the colum- 
ella and by a somewhat thicker shell. In comparing the recent species of Acteon, 
as lately monographed in Reeve’s Conch. Icon., Vol. XV, it must be agreed that no 
strict generic distinction between Acte@on and Solidula can be given. It is true, 
and can often be observed in the fossil forms also, that those species which have a 
double columellar fold generally are stout and strongly built shells, but at the 
same time there are among those with one fold, some which have an equally 
strong shell, though, as a rule, they are thinner. The distinction is therefore only an 
optional one, and in determining fossil species it is not easy to keep it up even 
within approximate limits. In fact JZyonia and Leucotina also could be considered 
as sub-generic divisions of Act@on, 
Species of Actg@on are known from the Trias upwards. 
Sb. TZornatellea, Conrad (Am. Journ. Conch. I, 1863, p. 145; Check list 
eocene fossils, 1866), The author quotes three species, of which Z. impressa 
(Acteon id., Gabb, Pal. California, I, p. 142) either belongs to the sub-generic 
eroup Solidula of Acteon, or more probably is an imperfect Ringicula. 
9.2 TFriptycha, Miller, 1859 (Suppl. to 1st and 2nd part of Monograph, Petreef. 
Aach, Kvreidef., Aachen, p. 21) with the type species 7. limneiformis, from the 
Senonien beds near Aachen. This is an ovate, almost perfectly smooth shell, with an 
aperture anteriorly rounded, possessing a sharp outer margin and an inner lip with three ’ 
folds, of which the middle one is the strongest. There was only one specimen 
found of this remarkable shell, associated with marine species, still I very much 
