INVERTEBRATE PALAEONTOLOGY. 455 



nodes that afterward elongate to form the costse. Ammonites proper also 

 presents differences in its septa, and never develops the characters seen in 

 the adult Prionotropis. In short, it would belong to an entirely distinct 

 family according to Professor Hyatt's classification. 



I have the impression that both sections of this group are confined to 

 the horizon of the Middle and Lower Chalk series. 



Subgenus PRIONOTROPIS, Meek. 

 PrioiKM jelns Woolgari, Mantell (sp.). 

 Plate 7, figs. 1, a-h ; and plate 6, fig. 2. 



Ammonites Woolgari, Mantell (1822), Geol. Sussex, 197, tab. xxi.fig. 16; and pi. xxii, tigs. 6 and 7.— 

 Sowerby (1829), Mill. Coucb., VI, 25, tab. 587, fig. 1.— D'Orbigny (1850), Prodr., 11,190 

 (not Paleont. Fr. Terr. Cret., I, pi. 108, rigs. 1, 2, and 3).— Sliarpe (1853), Fossil Rem. 

 Moll, found in the C'balk of England, 27, pi. xi, figs. 1 and 2.— Meek and Hayden (1801), 

 Proceed. Acad. Nat. Sci. Pbilad., 421.— Gabb (1801), Synop. Fossils Cret. Form., 14. 



Ammonites percarinatus, Hall aud Meek (1854), Mem. Am. Acad. Arts and Sei. Boston, V (n. s.), 390, pi. 

 IV, fig. 2 (young). 

 Compare J. Iiravaisiamts, and J. CaroKnns, d'Orbiguy (1840), Paleont. Fr. Terr. Cr^t.,I, pi. 91. 



Shell attaining a medium size, more or less compressed-discoidal, the 

 outer turn being proportionally more convex (including nodes) than those 

 within; umbilicus about equaling the greatest dorso-ventral diameter of the 

 last turn ; each volution embracing about one-fifth of the next within, and 

 having its umbilical margin slightly indented by the uncovered nodes form- 

 ing the inner of the two outer rows on the succeeding volution within. Young 

 examples, half an inch to one inch in diameter, with costse linear, closely 

 arranged, of nearly uniform size, and manifesting scarcely any tendency to 

 develop nodes, but already showing the forward curve of their outer ends 

 well defined, while the peripheral keel is low, narrow, and simple, and the 

 furrow on each side shallow. At a somewhat larger size, costse usually more 

 or less unequal in size, the larger ones now beginning to develop the two 

 nodes at their outer curved ends, and to become a little more prominent and 

 compressed at their inner extremities, while the rather more prominent keel 

 begins to develop its crenate outline, and the nodes nearest it to assume their 

 compressed form and parallel arrangement. On attaining to two and a half 

 to three inches in diameter, costse, nodes, and keel becoming more promi- 

 nent, the latter being strongly compressed and deeply and largely scalloped, 

 with divisions rounded in outline ; while at this stage of growth, the peri- 

 phery, as seen in profile, would seem to be very deeply sulcated on each side 

 of the keel, but this is due to the prominence of the row of nodes on either 

 side of the same. Costse, when the shell has attained a diameter of four 



