OF THE MESOZOIC MAMMALIA. 
215 
The result of this close comparison is first that, according to our present knowl- 
edge, the generic separation of PJagiaulax from Microlestes (type) is not very wide, 
and that the English species, M. Moorei, so far as known at present,^ cannot be sepa- 
rated generically from Plagiaulax, as it stands nearer this genus than it does to Micro- 
lestes. Second, we are justified in considering Microlestes as the earliest known 
representative of the Plagiaulaucidoe. 
PLAGIAULAX, Falconer, 1857. 
Type : P. BecMesU. Dentition, i 7 c 7 pm. 777 m. 7 This genus is so well 
known .through Owen’s and Falconer’s memoirs as to require no detailed description 
here.^ The different species mark numerous variations in the number of premolars, in 
the development of ridges upon their sides, and in the greater or less degree of 
confluence of the angle with the condyle. The more primitive of the Jurassic forms 
are those in which the premolars are four and faintly ridged, and the angle is en- 
tirely distinct from the condyle. The most primitive is Ctenacodon, Marsh, 
which may be considered a distinct genus,^ if we also subdivide the Purbeck 
species of Plagiaulax into two genera.'^ In this American form the condyle is pedun- 
culate, the angle is effected and inflected and the grooves upon the premolars are so 
faint as to be scarcely distinguishable. In the specimen of P. minor, (Professor Owen’s 
drawing, PI. lY. fig. 9, is more accurate than that in Falconer’s memoirs, A"ol. I. PI. 
33), unfortunately, the angular portion of the jaw is wanting ; it was probably inter- 
mediate between that of Ctenacodon and of P. medius ; there are two or three grooves 
upon pms 2^, while- there are about seven grooves on pm^, extending half-way 
across the crown. In P. medius the premolar grooves are much deeper and more 
numerous. 
The later dental evolution of the Plagiaidacidce is thus foreshadowed in the 
changes Avhich are observed in progress in the Jurassic species, viz, the loss of the an- 
terior premolars ; the growth and deepening of the ridges upon pm ^ ; the elongation of 
’ I judge from the specimen in the British Museum which seems to correspond closely to the figure of the 
molars in other collections. 
2 I may mention that I observed a faint cingulum upon the second molar of P. minor. Waterhouse observed 
the same (Xat. Hist, of Mamm.,) as cited by Dr. Falconer, Memoirs, p. 426. 
’Compare Marsh “Jurassic Mammals,” April, 1887. p, 332. The characters here assigned to distinguish 
Ctenacodon are: )“ four premolars instead of three.” In Falconer's type P. BecMesii there are but three pre- 
molars, but in the closely related P- medius (Owen), there is a socket for pm’, and in P. minor pm’ is well 
developed. ( 2 ) “ The summits of the teeth only are notched and the sides.smooth.” In a close examination of 
the premolars in C?e?iocodon very faint grooves can be observed on the sides. (3) “ The condyle has a distinct 
neck, see also Plagiaidax medius 'Fa\c. “ Memoirs,” p, 424, and Plate 34, Vol. II. (4). “ The outer margin is 
eflfiected ” see Owen, Mes. Mamm. p, 88. The constant presence of four premolars can be adopted as the 
generic distinction of Ctenacodon, which should naturally embrace P. medius and P. minor. 
* Professor Cope makes the P. minor of Falconer the type of a new genus, PUoprion, characterized by 4 
premolars ridged and serrate. Such a definition would also include P. medius, Owen, aud introduce further con- 
fusion, since P. medius is very close to P. Becklesii. 
