Arkell.1956.Jurassic..

502 MEXICO AND THE GULF REGION
taxonomic difficulty, and (in the lower part at least) by the peculiar
Oppeliid genus Mazapilites, not known outside Mexico (see Burckhardt,
1921, pis. i-iii, v-xiv, and list in Imlay, 1939, table 7). Imlay (1939, 1943)
has figured a number of new forms from these beds and made valiant
efforts to sort out the systematics of the Perisphinctids and the detailed
succession. It is still impossible, however, to place all the forms satisfactorily
in existing genera, or to draw a definite boundary between
Tithonian and Kimeridgian.
The presence of Lower Tithonian in the upper part of these beds,
probably above the range of Mazapilites, is proved by undoubted Virgatosphinctes,
correlating with the Lower Tithonian of Argentina and farther
afield (e.g. V. cf. denseplicatus Waagen sp., V. adkinsi Imlay, V. chihuahuaensis
Imlay (1943), V. aguilari Burck. sp., 1906 pi. xxvii, 6-9). The
predominant Perisphinctids, however, are a different group which seem
to fall for the most part into the genus Aulacosphinctoides (Burckhardt,
1921, pis. v-xiv, sub. Aulacosphinctes), characteristic of the Lower Tithonian
and Upper Kimeridgian of Spiti, New Zealand, etc. Many of the species
have a notable frequency of simple ribs, which strongly recalls the genus
Torquatisphinctes, and if, as Imlay holds, both genera are represented, it is
likely that these beds reach down into the Middle Kimeridgian. Another
pointer in the same direction is the occurrence of forms of Perisphinctids
(P. mexicanus Burckhardt, 1906, pi. xxxi, figs. 6-9) which appear to belong
to the genus Virgataxioceras of the Beckeri Zone, and the overlap, according
to Burckhardt, of Mazapilites and Hybonoticeras. Other forms occur
which Imlay (1943, pp. 532-3, pis. 88, 89, 91) assigns to the late-Middle
and early-Upper Kimeridgian genus Subplanites, the genus to which P.
burckhardti Blanchet (1923) probably belongs (type Burckhardt, 1921,
pi. xiv, figs. 1-3); and 'Perisphinctes nikitini' Burckhardt (1906, pi. xxxi,
figs. 1-4) seems to be a Subdichotomoceras. Another fragment figured by
Burckhardt (1906, pi. xxxii, fig. 2) may be identical with an early Virgatosphinctes
of the Neuburg Lithographicum Zone, V. eystettensis Schneid
(1914, pi. iii, fig. 5). With these forms occur a considerable variety of
Aspidoceras and Physodoceras, Taramelliceras, Pseudolissoceras, Haploceras,
etc. It is difficult to believe that careful stratigraphical collecting will not
one day yield a sequence and solve the problems.
Some ammonites, mostly indeterminate fragments, that may represent
a different Upper Kimeridgian fauna of the Pectinatus Zone have been
figured from an isolated outcrop near Las Cuevas in eastern Durango
(Imlay, 1939, p. 33-4). They have been assigned to Callovian genera
and dated (p. 21) to the 'Middle Oxfordian', but they bear a suggestive
resemblance, in the photographs, to Pectinatites (pi. 7, fig. 7) and Wheatleyites
(pi. 7, fig. 1; pi. 5, fig. 8; pi. 6, fig. 1; pi. 8, figs. 1, 2).
MIDDLE AND LOWER KIMERIDGIAN
The Middle Kimeridgian is represented most recognizably by beds with
Hybonoticeras cf. beckeri (Neum.), H. cf. harpephorum (Neum.), H. cf.
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