Part II - State of New Jersey

AMMONOIDEA 129Placenticeras placenta (DeICay). Johnson, 1903, Columbia Univ. SchoolMines Quart., vol. 24, p. 133, pl. 7, figs. 26a, h.Placenticeras placenta (Delcay). Shimer and Blodgett, 1908, Am. Jour.Sci., 4th ser., vol. 25, p. 65.Placenticeras placenta (DeKay). Dowling, 1917, Canada Geol. SurreyAIem. 93, p. 32, pL 34, figs. 1, la.Placentieeras placentu. (DeKay) var. il~tcrcalarc (Meek). I)owling,idem, p. 32, pl. 33, figs. I, la.Placenticeras placenta (DeICay). Roman, 1938, Les ammonites jurassiqueset cretacGes, p. 504, pl. 52, figs. 487, a.Description.-This species attains a large size, individuals approaching500 mm. having been recorded. The venter in the largest individualsis rounded, but even at a large diameter the ventral trnncationcharacteristic of the genus is still observable. The shell at allstages is relatively compressed, the flanks gently convex, the umbilicusvery small. Whitfield states that he saw no nodes on the flanks, butRlorton records umbilical tubercles and Hyatt writes of three rows oft~ribercles on smaller specimens, umbilical, two-thirds the distance outfrom the umbilicus on the flank and distantly spaced, and on the marginof the venter. Hyatt describes the ventral tubercles as large andelongated parallel to the venter; Weller clescribes them as alternate.The larger individuals are essentially smoot,h. The suture is much dissected,with long elements, ranging from long and narrow to moresolid and rounde(1. The ar~alogons species in the Westcrn lntcrior isP. nleeki Hoehm (zuhitfieldi ETyatt), hnt mceki is more compressed, theventral truncation is narrower and lilore persistent, nodes are lackingon the flanks, and the suture is more con~ples.Rc~i~arks.-Much confusion has existed in the literature becanse ofa long-standing tencleney to assign all species of Placentieeras to plncenta.IIyatt's work has helped to correct this tendency, but bcca~~scof his habit of discussing rather than (lcscribing features? some of it isdifficult to follow, The species place~~tr~ is relativelp frequent in theolrler horizons, the Nerchantville clay-according to \fTellcr-provirlingthe most and best specimens. Those from othcr horizons arc morefragmentary and smaller.The specimen here illustrated is accompanied by a label indicatingt,hat it was figured by Ibforton in his Synopsis (pl. 2, fig. 1). Out of handthe specimen does not look like Morton's figure, as the latter shows nodefects and the specimen lacks all but the basal part of the living chamber,but Morton's figure is apparently much restored (as are those ofAmmonites vannxemi on the same plate, for which therc can he noquestion as to the original specimen). In size it fits the septate partof Morton's figure very well, for Morton says the specimen was 15inches in diameter, including a large part of the living chamber. Theopposite side of the specimen here ill~istrated was figurerl by Whitfield(his pl. 40, fig. 1).NEW JERSEY GEOLOGICAL SURVEY