[3] The specimens in which I have seen the vertebral articulation show no co-ossification of the vertebrae: the facet for articulation being placed above the spines, and apparently formed by ossified ligaments. In view of the above, the practice of the American text-books in Geology in introducing generic names of characteristic fossils as names of the geological horizons whence they come, is very reprehensible, in my opinion. Even the late edition of Leconte’s Elements contains a long list of such names, the greater portion of which have been relegated to the limbo of synonymy by paleontologists. It is greatly to be desired that the name “Pteranodon Beds” shall not become established, so long as there is the least doubt of the validity of the name itself. KANSAS MOSASAURS. BY S. W. WILLISTON AND E. C. CASE. PART I, CLIDASTES, WITH PLATES II-VI. The group of extinct Cretaceous reptiles known as the Mosasaurs or Pythonomorpha was defined by Cope, “to whom Science is so largely indebted for its present knowledge of this interesting order of reptiles” (Marsh), in 1869.[4] Although some of the characters assigned by him to the order have since been shown to be inapplicable, and the group to have less value, yet his name, Pythonomorpha, has been generally retained. Lydekker and Zittel have assigned to the group a subordinal value, as has also Marsh, though under a different name. Owen rejected it entirely, and Baur, more recently,[5] has united it with the Varanidae to form a super-family, as follows: Suborder Platynota. Super-family Varanoidea. Families Mosasauridae, Varanidae. Super-family Helodermatoidea. Family Helodermatidae. The group, whatever may be its rank or position, includes, so far, the following genera: _Mosasaurus_ Conyb., _Liodon_ Owen, _Platecarpus_ Cope, _Clidastes_ Cope, _Baptosaurus_ Marsh, _Sironectes_ Cope, _Plioplatecarpus_ Dollo and _Hainosaurus_ Dollo. _Pterycollasaurus_ Dollo, founded upon _Mosasaurus maximilianus_ Goldf., is omitted as doubtful. All of these genera, save _Plioplatecarpus_ and _Hainosaurus_, have been recorded from North America, _Clidastes_, _Baptosaurus_ and _Sironectes_ being peculiar to this country. Of these latter three genera, however, _Clidastes_ alone is well known; but this genus is suspected by Lydekker of being the same as the imperfectly known European _Geosaurus_ Cuvier. Thus it seems that the genera, or at least the most of them, have a wide distribution; _Platecarpus_, in fact, is said to occur in New Zealand. [4] Proc. Bost. Soc. Nat. Hist., p. 253. [5] Science, xvi, p. 262, Nov. 7, 1890.