20 MALONE JURASSIC FORMATION OF TEXAS. [bull. 266. 
gonia prcestriata — represented by a single somewhat imperfect 
mold — none of the Ma lone Trigonias agree perfectly with generic 
sections hitherto known from Cretaceous rocks, exclusively. The 
genus Trigonia is moreover generally considered one of the best of 
the Lamellibranch genera for purposes of stratigraphic diagnosis. 
But it is to the ammonites that we must look for the strongest 
evidence of geologic age, these being generally regarded as the most 
critical of molluscan fossils as guides to stratigraphic correlation. 
And Jiere, too, the evidence is clearly conclusive of Jurassic age. 
Though several of the genera are common to the Jurassic and Cre- 
taceous systems, all of the known European analogues in species and 
subgeneric groups arc Jurassic. The ammonitic genus of most fre- 
quent occurrence in the Malone is Perisphinctes, which is common 
to the two systems, but is almost exclusively Jurassic. Of the 366 
known species of this genus b only four, described by Neumayr and 
Uhlig from the upper Xeocomian of Saltzgitter, c are Cretaceous; 
and these belong to a group which the Perisphinctes species of the 
Malone formation do not at all approach. On the contrary, the 
species of Perisphinctes to which those of Malone show the closest 
affinities are those of the Tithonian. 
It should be noted that while some of the molluscan fossils other 
than ammonites seem to indicate Kimmeridgian, or earlier than 
Tithonian age, this is more or less offset by the presence of the ordi- 
narily Cretaceous genus Ptychomya. 
LOCALITIES OF RELATED FAUNAS. 
The only localities which present rocks known to correspond more 
or les^ closely with those of the Malone formation are in Mexico. 
In the descriptive part of this paper it is noted that some of the : 
fossils of the Malone formation are identical with some of those of 
the Alamitos beds, described by Castillo and Aguilera as occurring 
at the Sierra de Catorce in San Luis Potosi, and that others agree 
with species described by Doctor Felix from the Cerro de Titan ia in 
Oaxaca. The number of forms at present known to be common to 
the Malone and the Alamitos is so considerable that there can be 
little doubt that these two names represent approximately the same 
horizon. The ammonites of the Alamitos, like those of the Malone, 
are of Titonian affinities. 
a Doctor Stanton, however, places Trigonia proscabra in the Scabrae, and questions 
whether T. calderoni be not referable to that section also rather than to the Undulate. 
In the body of his recent Monograph of Perisphinctes, Siemiradzki treats 363 sp< <■ cles 
of this genus. In its supplement, of a large number of additional alleged new species 
cited as described by De Ilias and Tornquist in 1898, he identifies all but three with 
species previously known, so that the number of species recognized by Siemiradzki in 
the early part of 1809 was 366. 
c Ueber Ammoniten aus den Hilsbildungen Norddeutschlands ; Paleontographica. Bd. 
XXVII, 1881. 
