102 MALONE JURASSTC FORMATION OF TEXAS. [bull. 266 
To the above description Castillo and Agirilera add the following 
remarks : 
We place this species in the genus Placenticeras, notwithstanding that by | r . 
its form it could be considered as a Phylloceras, by the characters of the 
sutural line which, though imperfectly visible in one of the specimens of our 
collection, does not present the saddles terminated by the roundels characterise 
of the Phylloceras, only ramifications and subdivisions similar to those of the 
Placenticeras. 
As it makes some departures from the characters belonging to most 
of its congeners, this ammonite may seem at first sight almost as 
much of an anomaly in the genus Oppelia as it was supposed to be irln 
Placenticeras. But the suture, which has the lobes and saddles very 
few instead of many," and has the first lateral lobe contrastingly 
the largest, sufficiently distinguishes it from the latter genus and! 
relates it to Oppelia and Haploceras, with which also agree its invo- 
lute, narrowly uriibilicated shell, its rounded venter, its rather com- 
pressed lenticular form, and the absence of periodica] constrictions. 
It seems indeed to have its closest affinities and to belong among the 
round -ventered, phylogerontic forms of Oppelia which occur in the 
latest Jurassic rocks, in which this genus makes its final appearance! 
the venter, in 0. fallax, being rounded both on the outer and on the 
inner n horls. 
The Malone Oppelia bears considerable resemblance to the Titho- 
nian species. 0. waageni Zittel, having about the same outward form 
and being of large size; but it apparently lacks a vestige of even such 
feebly expressed, coarse lateral ribs, or undulations, as are seen on 
that shell, and is more critically distinguished from it by the charac- 
ters of the suture. The first lateral saddle is much narrower in the 
Malone ammonite than in Oppelia waageni, and the (irst lateral 
lobe — which is quite unsymmetricalry branched in the latter shell — 
presents an almost symmetrical branching in 0. fallax. In this last- 
mentioned character, 0. fallax departs from the condition most com- 
mon in Oppelia and approaches the usual condition in Desmoceram 
Prom which genus it is separated by the absence of constrictions. 
The siphonal lobe is about half as long as the first lateral lobe and 
ends in two small branches on either side. 
Though commonly represented from the Malone district by whorl 
fragments of dimensions not or little exceeding those given by Cas- 
tillo and A.guilera for the examples from the Sierra de Catorce, tlr 
shell attains a much Larger size. One specimen, of which consider- 
able portions were collected \ }, miles east of Malone station, has in 
the largest preserved cross section of the body chamber, a height of 
about 145 and a breadth of at least 93 mm., while the diameter of 
" My attention was first called to the few-lobed suture, as inconsistent with PlaccnticeraM 
by Doctor Stanton. 
