270 DESCRIPTION OF A COLLECTION OF FOSSILS, 



with concave interspaces nearly as large as the ribs. These begin at the suture, 

 inside of the umbilicus, are inclined slightly forward, flexed, and near the dorsum 

 are bent forward with a broad gentle curve, terminating against the sharp dorsal 

 carina. Aperture elongate ovate ; septum unknown. The greatest change that 

 occurs with age in this species is in the dorsal carina. As the shell grows older, 

 this becomes higher and narrower until it assumes the character of a prominent 

 plate. The ribs disappear as they reach its base. See Figure 2 b. 



Measurements. From a small specimen. Greatest diameter, 2.9 in. ; length of 

 aperture, 1.4 in.; width of aperture, 0.45 in.; height of aperture above the 

 dorsum of preceding volution, 1.0 in. 



Locality. From the Liassic {%) coal mine of Pariatambo, associated with 

 Myophoria, and other characteristic forms. Also from the " Cerro de la Ventanilla," 

 at a height of 5000 metres. 



Remarks. From A. primordialis, Schlot., of the Upper Lias, this shell differs in 

 being less acute on the dorsum, in having a smaller umbilicus, and more flexed 

 ribs. It is more convex than A. complanatus, Brug., of the same formation, and 

 the ribs want the backward flexure of that species. In both these respects it also 

 differs from A. discoides, Ziet., also an Upper Lias shell. The surface markings 

 are not unlike A. (?) Feruvianus, Von Buch, a Cretaceous fossil, probably not an 

 Ammonite, since the whorls do not seem to envelop their predecessors. . The present 

 species is deeply enveloping. In Von Buch's figure, too, the ribs are represented 

 as twice as wide as those of our fossil. 



A. BiLOBATXJS, Hyatt, PL 38, fig. 3, 3a, 3b. 



Buchiceras bilobatum, Hyatt, Proc. Bost. N. H. Soc. vol. 11, p. 3T0. 



Shell broad, robust, whorls increasing rapidly in size, and deeply enveloping ; 

 umbilicus deep, less than a third of the diameter of the shell ; sides flattened and 

 slightly converging ; dorsum broad, flattened, and bearing a broad, but not high, 

 median rido-e ; aperture nearly square with the corners rounded and the dorsal 

 face a trifle the narrowest. Surface marked by a few very large ribs beginning 

 just inside of the umbilical angle, slightly tuberculated on the angle and running 

 to the dorsal margin, where they carry another small angular tubercle. An occa- 

 sional supplementary rib is interpolated on the dorsal half of the side. Septum 

 consisting of a dorsal lobe, two laterals on the side of the whorl, and two on the 

 umbilical face. Dorsal lobe broad and short, and lying between the two rows of 

 tubercles; at the corners it ends in two short slender branches, simple on their 

 inner face and tridentate outside ; between them is a broad emargination, its base 

 nearly straight, and bearing six small teeth ; above the branches are two large 



