INVERTEBEATE PALEONTOLOGY. 449 



gined externally by a row of compressed nodes; umbilicus wide ; volutions 

 narrow, slightly embracing, and ornamented by regular, simple, straight, 

 tuberculated costne. Septa in the typical species with three lateral lobes on 

 each side, the first one being longer than the siphonal lobe, with tripartite 

 extremity, the terminal division being deeply bifid ; second and third lobes 

 much smaller and more or less tripartite or dentate ; first and second lateral 

 sinuses more or less nearly equally bipartite or bilobate at the ends. 



Shells of this genus will be distinguished from the restricted genus 

 Ammonites by their always single peripheral keel, straighter and differently- 

 tuberculated costse, and differences in the forms and proportions of its septa 

 lobes and sinuses. It is apparently restricted to the Cretaceous system, 

 while the genus Ammonites, as represented by its typical forms, is probably 

 confined to the lower members of the Jurassic system. 



Compared with the types for which I have proposed the names Priono- 

 cyclus, it will be found even more strongly separated by its simple (nou- 

 crenate) peripheral keel, straight, more regular, and diflferently-tuberculated 

 costae, &c. It will include, in addition to the type-species T. vespertinus 

 (— A. Texanus), one, if not both, of the species hereinafter referred provis- 

 ionally to it; also, perhaps, Ammonites Leonensis, Conrad; and possibly an 

 East Indian form, referred by Dr. Stoliczka to A. inflatus, Sowerby, as well 

 as some other Cretaceous species. It seems to be confined to the Creta- 

 ceous system. 



Itl o r t o n i c e r a s Shoshonciisc, Meek. 



Plate 6, figs. 3, a, c, and (i, h.* 



• Shell compressed-discoidal, with umbilicus apparently nearly or quite 

 twice as wide as the outer whorl ; volutions very narrow, with dorso-ventral 

 and transverse diameters equal, and section subquadrangular, those within 

 scarcely one-sixth embraced by the succeeding turn ; costs; each mainly 

 represented by two nodes, the inner of which is low, compressed, and elongated, 

 so as to extend from near the umbilical margin about half-way across the 

 sides, while the outer near the peripheral margins is more prominent, rounded, 

 and directed laterally ; keel' less prominent than the row of compressed 

 nodes on each side about half-way between it and the rounded nodes along 

 the margins of the periphery ; compressed nodes on the periphery of each 



* Figure 3, 6, of this species, was by error numbered li, h, on plate 6 ; while a figure of a small 

 Scaphites farther flown on the same plate is also numbered fl b. 



57 H 



