2t4 DESCRIPTION OF A COLLECTION OF FOSSILS, 



with a marked inclination backwards; these afterwards curve forwards on the lower 

 half of the whorl, from the middle continue transverse to the dorsal angle, and 

 then incline strongly forward, forming an acute angle between the two sides against 

 a large median dorsal keel. At the dorsal angle each rib is developed into a large 

 flattened tubercle placed at an angle of about 45° to the corresponding one on the 

 opposite side. Below this large tubercle, on each rib, is a smaller one, placed on 

 the middle of the upper half of the side of the whorl. Septum unknown. 



Measurements. Height of aperture, 1.5 in.; width of aperture, 1.1 in. 



Localities. Liassic; one adult fragment from the "limestone of the Cerro del 

 Ventanillo, between Pachachaca and Jauja. Height of 5000 metres." Another, 

 and a very young shell, from the " coal mine of Pariatambo, 5 leagues from 

 Morococho. Height 4000 metres." Dr. E,aimondi considers both these localities 

 Jurassic, and remarks of the former that the formation is very extensive. Still 

 another specimen, a little over an inch in diameter, is from the " neighborhood of 

 Fingo, Prov. of Huari, Dept. of Huaraz, from a schist containing coal." 



Remarks. Our species is very closely allied to the Liassic form, A. spinatus, 

 Brug., and may eventually prove to be identical, since they only differ in details 

 of ornament, both going through the same series of changes from youth to the 

 adult stage. In the specimens before me, however, the characters on which I have 

 depended for a separation are constant. There is a broad concave space on each 

 side of the keel, between it and the tubercles, the space between the outer sides 

 of which is almost as great as the greatest width of the aperture. In A. spinatus 

 these tubercles are placed entirely on top, and the second row, which in our shell 

 is well down on the sides, forms the outer margin of the dorsum in that. Further, 

 in.our shell the ribs on the ventral half of the whorl are markedly flexuous, while 

 in A. spinatus they are straight. 



A. GiBBONiANUS, Lea, Tr. Amer. Philos. Soc, 2 S., vol. Y, p. 254, pi. 8, f. 3. 



A single fragment, not more perfect than Mr. Lea's specimen, occurs in the 

 collection from " between Huandoval and Coronojo." It shows no additional 

 characters, being a piece of one whorl, about as long as broad, and having but four 

 ribs and part of another. It is more convex in its cross section than the form 

 figured by Marcou. Mr. Lea's original, now in the museum of the Academy of 

 Natural Sciences, although weathered on one side, shows that it had a rounded 

 dorsum. The shell from Texas referred to this species by Marcou (Geol. of N. A., 

 p. 35, pi. 2, f. 2), has an acutely carinated dorsum, and the ribs are acute, while 

 the South American has rounded ribs. These ribs in Gibbonianus run all of the 

 way to, and apparently cross the dorsum. In the Texan they stop short. The 



