PLACENTICERATID ®. 243 
Placenticeras. This genus therefore combines the external characters of 
Engonoceratidee with the sutural lines of Placenticeratide, and perhaps 
ought to be placed in a separate family. 
The young, however, are unknown, and it is perhaps better to wait 
until their development places their affinities on a firmer basis. It is also 
possible that they may be accounted for as members of the Placenticeratidze 
that were arrested in their development, retaining the neanic condition of 
the venter and lateral zones in their later stages, but not arrested in their 
sutures, which approximate to those of Placenticeras. 
DIPLACMOCERAS BIDORSATUM (Roemer). 
Ammonites bidorsatus Roemer, 1841, Verst. nord. Kreidegeb., p. 88, pl. 13, fig. 5. 
This species has an outer row of lateral tubercles, according to 
Roemer’s figures, with an inner row of folds or costee directed apicad. The 
outer row of lateral tubercles are elongated continuations of the coste and 
are bent orad, forming crest-like tubercles near and parallel to the edges of 
the venter. This last is a concave zone, bordered by two continuous ridges 
without tubercles. The outer volution is decidedly compressed, but is 
rather stout in proportion to its dorso-ventral diameter. This species is 
obyiously, unless both the figures and description are erroneous, quite 
distinct from the bidorsatum of Schliiter, in having stouter volutions and 
broader venter and in the inner lines of narrow folds. There is, however, 
considerable resemblance between the two, and it is unlucky that Roemer 
did not figure the sutures. 
Age: Lower Senonian. 
DIPLACMOCERAS CANALICULATUM n. sp. Hyatt. 
Ammonites polyopsis Schliiter, 1867, Ammoneen norddeutsch. Senon., pl. 4. 
Ammonites bidorsatus Schliiter, 1872, Paleontogr., Vol. X-XI, pl. 15, figs. 6-8. 
This species is very peculiar, having a well-defined gerontic stage with 
a row of lateral tubercles* set well out near the venter and becoming 
nodose or prominent. The shell in the ephebic stage is smooth and 
compressed, the venter is extremely narrow and channeled, and there are 
aThese are in no way comparable with ventral lines of tubercles as they occur in Placenticeras, 
being on the sides and near, but not on, the edges of the venter. 
