XIII.—The Genera Dictyoconoides Nuttall, Lockhartia nov., and Rotalia Lamarck: Their Type Species, Generic Differences, and Fundamental Distinction from the Dictyoconus Group of Forms

The author recalls the facts regarding the rediscovery, in recent years, of Carter's genus Conulites ( = Dictyoconoides Nuttall). He shows how, of seven species which have been referred to this genus, three always have numerous intercalary whorls in their spires, and four never have any such whorls. The association of these two groups together within a single genus has, so far as he himself is concerned, been due to the uncertainty hitherto existing regarding the type species cooki, whose spire was stated by Carter sometimes to show an intercalary whorl, but generally to be “single throughout.” Carter's original specimens, which had long been mislaid, have recently been found, however, and they all prove to belong to now familiar types, which invariably contain numerous intercalary whorls in their spires. The author therefore separates the four species which do not possess such whorls into a new genus, Lockhartia.The author next discusses the supposed identity of Dictyoconoides with Rotalia, and shows that the Rotalia must be judged by their type species R. trochidiformis. He shows that this species, among other characteristic features, invariably displays the structures called “astral lobes” by Carpenter and Brady, and is distinguished by the same from both Dictyoconoides and Lockhartia; while it is further distinguished from Dictyoconoides by the absence of intercalary whorls in its spire.The author then compares the development of Rotalid forms, like the above, with that of piano-spiral ones like Nummulites and Assilines; and he finally shows the fundamental difference in structure between all these spirally wound types and those with an “end-on” development, like the Dictyoconus group of forms, with which some of them have been confused in the past.