| OCR Text |
Show 370 from Nebraska, as well as those found in New Jersey, do not show any essential variations in this respect. In the relative size, number, and arrangement, as well as in the mode of branching, of its lobes and sinuses, the shell under consideration agrees very nearly with authentic specimens of A. complexu* ; though in the details of the divisions of its septa there are differences from those of that form. Still these differences, as may be seen from the figure given on plate 5, seem to be of such a nature that they may be due to different degrees of development in the specimens compared; the few individuals of A. complexu* yet found, either at the original locality or in New Jersey, being smaller than those of the form under consideration with which I am comparing them. That first figured by Professor Hall and the writer is not in a condition to give a clear idea of the septa of the Nebraska shell In the lobes and sinuses of its septa, this species is also very closely allied to A. Gollevillensis, d'Orbigny (= A. Lvwisienzis, d'Orb., not Sowerby). It is a more gibbous shell, however, and differs in having its coatee developed entirely across to the inner side of its whorls, while its periphery is completely destitute of the longitudinal groove so characteristic of d'Orbigny's species. It is also nearly related to A. Egertonianus, Forbes, from the Cretaceous of India, but its volutions are less compressed, its costo smaller, and its septa different in some of their details, particularly in the arrangement of the small inner lobes near the umbilical margins. The largest specimen in the collection is from Sucia Islands. It is an internal cast, and has its outer turn so much worn as to obliterate much of the details of the lobes and sinuses, as may be seen by fig. 2 b of plate 5. The inner turns, however, when separated, show the septa very clearly. It consists entirely of septate whorls, the non- septate portion having been broken away. In its greatest diameter, it measures 4.60 inches, and 1.90 inches in breadth. Others ( retaining most of outer chamber) measure about 3.15 inches in their greater diameter. Locality and position.- Eomooks, Vancouver's Island, and at Sucia Islands; Cretaceous. Genus PLACENTICERAS, Meek. PLACENTICERAS ? VANCOUVEBENSE, Meek. Plate 6, fig. 1,1 a, 1 ft, and 1 c. Ammonites Vancouverensis, Meek ( 1861), Proceed. Aead. Nat. Soi. Phila., xiii, 317. Shell discoidal; volutions increasing rather gradually in size, strongly compressed on the sides, and flattened on the narrow periphery, nearly twice as broad from the peripheral to the umbilical side as the transverse diameter, and rather deeply embracing; umbilicus of moderate depth, and small, or rather less than half as wide as the greater diameter of the outer whorl; aperture ( as nearly as can be determined from a section of one of the whorls) compressed- subcordate. Surface ornamented by a row of compressed nodes along each margin of the periphery, and another of smaller size around the umbilicus of each side; about twenty of the first and ten of the latter may be counted on each side of an entire turn. The flattened sides between the peripheral and umbilical rows of nodes are provided with obscure, slightly- arching, transverse cost ® , one of which extends from each of the umbilical nodes toward the peripheral margin; but all become obsolete before reaching it, at least on the outer turn. Between each two of these principal |
