ADDITIONAL remains of mammals from the Jurassic of the Rocky Mountains indicate that this class cOllstituted an important element in the Mesozoic fauna of this country. The fOl'ms already described,* as well as those noticed below, show moreover, such a resemblance to known t.ypes from the Purbeck of England, that some connection between the two faunre is clearly implied, and future discoveries will be awaited with interest. Otenacodon serratus, gen. et sp. nov. One of the most interesting

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... mens yet brought to light is a diminutive right lower jaw, witn most of the teeth in excellent preservation, This specimen differs widely from the remains hitherto found in tbis country, but agrees in its main features with the genus Pl(~gialllax of Falconer. t Fl'Om the type species of that genus (P. Beckelsii), it differs in having four lower pI'emolars instead of three; while from all the described species, it may be distinguished by the absence of the chal'acteristic oblique grooves on the sides of the premolal' crowns. Thii:! specimen is represented in the figure given below. Right lower jaw of atenacodon serratus, Marsh; about four times natural size. a. incisor; b. condyle; c. coronoid process.