MORRISON MAMMALS: MULTITUBERCULATA 13 



Suborder PLAGIAULACOIDEA Simpson 1925 



Definition. — Molars two in number, the first, at least, elongated. The lower 

 molars with two and the upper molars with two or three longitudinal rows of two to 

 twelve cusps each. Premolars sharply differentiated from molars, the posterior ones 

 forming a strong shearing device, secondarily lost in the Taeniolabididae. The lachry- 

 mal reduced or absent, the jugal and frontal much reduced, with progressive overlap- 

 ping of the latter by the parietal. Zygomatic process of maxilla strong, with a squamo- 

 sal contact. Palate usually broad, extending back to end of molar series, with or 

 without vacuities. 



This suborder is practically coextensive with the family Plagiaulacidae of some 

 authors, but its right to rank as a suborder can hardly be disputed if the same criteria 

 are applied as are used in classifying Tertiary and recent mammals. From a strictly 

 Linnaean (or static) point of view, its morphological variety and distinctness is as 

 great as in the average suborder of conservative recent classifications, and from a 

 phylogenetic (kinetic) point of view, the relative time of its known emergence, the 

 number of included phyla, and the plan and scope of its development also fully war- 

 rant its subordinal separation. There is a human tendency to lump together under a 

 few easily remembered names a number of really quite diverse units when the latter 

 are not well known to the individual taxonomist. There is also, of course, the tendency 

 of the specialist to multiply terms and to give his own groups a high status which 

 would be impractical in a more general view of the class, but the Mesozoic mammals 

 have certainly suffered from the former tendency and not from the latter. Recording, 

 as they do, twice as long a period of evolution as the Tertiary mammals, they are 

 worthy of emphasis, and, as this memoir is intended in part to reveal, their remains 

 although scanty and imperfect are more characteristic than is generally believed. All 

 that is asked is that some approach be made toward applying to them the same taxo- 

 nomic standards as are applied to other mammals. 



Four families are provisionally referred to the Plagiaulacoidea. One, the Micro- 

 cleptidae,' is wholly doubtful as to the affinities and is placed here only provisionally. 

 The Taeniolabididae ("Polymastodontidae"), recognized as a family by Cope and 

 still often so classified, were reduced to subfamilial rank by Osborn. The group is now 

 recognized as a family, but it is exclusively Paleocene and is not here treated. There 

 remain a number of upper Jurassic, Cretaceous, Paleocene, and Eocene genera which 

 have hitherto, almost without exception, been retained by all in the family Plagiaulaci- 

 dae, and even in a single subfamily Plagiaulacinae. They include, however, two quite 

 distinct groups, one upper Jurassic and lower Cretaceous in age and typified by Plagi- 

 aulax and Ctenacodon, the other ranging from upper Cretaceous into the Eocene and 

 tj-pified by Ptilodus. These may be contrasted as follows : 



' Microlestidae of earlier workers. The generic name Microlestes was preoccupied when applied to 

 a member of this group. See Simpson 1928A. 



