38 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



Placenticeras, but is regarded by Kossmat as representing a distinct group 

 related to his Neoptychites and also showing some affiliation with Hop- 

 litcs leofwldinus. But, although there is also similarity in the amount of 

 involution and in the form of the umbilicus, the differences in the gen- 

 eral form of the shells, especially in the ventral region, and in the details 

 of the sutures, show that the species referred to Hatchericeras are not 

 congeneric with Amm. clypeiformis. The conclusion drawn from all 

 these comparisons is that the Patagonian ammonites described herein 

 have an assemblage of characters not found in any described group, and 

 they are therefore assigned to a new genus, whose affinities are closest 

 with the Hoplitidae, though Hyatt's 1 definition of that restricted family is 

 hardly broad enough to include it. 



HATCHERICERAS PATAGONENSE sp. nov. 



PI. VIII, Figs, i and 2, PL IX, Fig. i. 



Shell large, involute, compressed, with very slightly convex sides that 

 show an obscure flattening between the middle of the whorl and the 

 umbilicus ; venter regularly rounded ; umbilicus about one fifth the diam- 

 eter of the shell, funnelform, the shoulder broadly rounded and the umbil- 

 ical wall of each whorl slightly impressed or concave in the middle ; surface 

 of adult whorls marked only by rather coarse irregular growth lines that 

 are seen when the shell itself is preserved but leave no trace on the internal 

 cast. These growth lines show slightly in the umbilicus and are most 

 conspicuous on the middle of the flank, where they form rather coarse, 

 slightly curved wrinkles that may indicate the position of a lateral ear or 

 lappet, such as are frequently seen in Hoplites and other groups of ammo- 

 nites. The figured type gives no indication of the sculpture of the young, 

 as its umbilicus could be cleaned only enough to show about one and a 

 half whorls, but the smaller specimen, showing about three whorls in the 

 umbilicus, bears on the umbilical margin of the inner whorl the ends of 

 rather distant prominent ribs (or a row of elongated tubercles) closely 

 resembling those seen in the umbilicus of H. tardense. 



The suture has been sufficiently characterized in the generic description 

 and is figured natural size on plate IX. The other figures of the type 

 specimen are one half natural size. 



1 InZittel's Text-book of Palaeontology, Vol. I, pt. II, p. 584, Macmillan & Co., 1900. 



