Revieics — Fossils of the Sierra Catorce, Mexico. 523 



summit, which is elevated 1200 metres above the plains of San Luis. 

 It is a well-known mining area. The Sierra consists of three prin- 

 cipal rock divisions : the lowest is formed of altered shales or 

 phyllites, in which no fossils have as yet been discovered ; the 

 second division, which is rich in fossils, is built up of arenaceous 

 and marly shales; whilst the upper division consists of a shaly lower 

 portion, and over this compact gray limestones with nodules and 

 bands of chert or flint. This upper division contains species of 

 Exogyra, Lncina, Phylloceras, and Schloenbachia, which are considered 

 by the authors to represent the Aptian and Albian stages of the 

 Lower Cretaceous. 



The main purpose of this memoir is the description of the fossils 

 of the second division, which are regarded as in part of Jurassic 

 age, and in part Cretaceous. The authors subdivide the formation 

 into higher and lower groups. The higher, or Cieneguita, is of shales 

 and sandy marls, more or less calcareous, which contain no fewer 

 than nine species and varieties of Aucella, together with Lytoceras, 

 sp.n., Placenticeras, sp.n., Fulchellia, sp.n., and Olcostephanus, sp.n. 

 The most important and numerous of these fossils are the various 

 species of bivalve mollusca of the genus Aucella which are inter- 

 mingled in the different strata of this subdivision, the thickness of 

 which altogether does not exceed five metres, whilst in Russia five 

 of these same species, according to Lahusen, characterize as many 

 diiferent zones of the Upper and Lower Volgian ! As the Mexican 

 Beds contain as well a form of Olcostepliamis, which is nearly iden- 

 tical with 0. Asterianus, D'Orbigny, from the Neocomian, and 

 a species of Pulchellia, a genus exclusivelj^ Lower Cretaceous, the 

 author thinks that the higher group with Aucella should be regarded 

 as Neocomian. 



The lower subdivision, or the Alamitos group, consists of fine- 

 grained sands and argillaceous shales. It contains BhyncJionella, 



2 sp., Terebratula, 2 sp., Waldheimia, sp.n., Aucella Broiini, Cucullcea, 

 sp.n., Lucina, 2 sp.n., Cyprina, sp.n., Cyprimeria, sp.n., Goniomya. sp.n., 

 Pleuromya, sp.n., Vermetus ? sp.n., Nautilus, sp.n., JlJiacopliyUites, 



3 sp.n., Haploceras, 3 sp.n., Perisphinctes, 16 species, of which 12 

 are new, Olcostephanus, sp., Ifoplites, 4 sp.n., Aspidocerns, sp.n., 

 Aptychus, 2 sp., and Belemnites, 2 sp. Reviewing the affinities of 

 the various forms in this group, it is placed as Upper Jurassic, 

 probably on the horizon of the Kimeridge and Portland Beds. 



The descriptions of the fossils in this memoir ajipear to have been 

 carefully drawn up, but the figures are not sufficiently clear to allow 

 of any safe judgment to be formed as to the characters of the 

 species they are intended to represent. Ttiis is the more unfortunate 

 since such a large proportion, more particularly of the Ammonites, 

 are described as new. As regards these latter, it may be remarked 

 that in no single instance are the suture-lines figured, and as they 

 are only referred to in one or two cases, it may be supposed that 

 they are not shown in the type-specimens. Again, it would be 

 impossible to discriminate from the figures the various species of 

 Aucella, and unless the originals show the specific characters clearer 



