RHiETIC FORMATIONS. 5 



Basilosaurus of Harlan is shown not to aflPect the rule of two roots, being exclusively 

 mammalian,^ concluded that he had evidence of a Mammal from that Mesozoic group of 

 strata now termed ' Rhsetic,' which is older than the Oolitic or Jurassic group, and he 

 proposed for the small species represented by the two teeth the name Microlestes anti- 

 quus," rightly referring it, notwithstanding the diminutive size indicated by the fossils, to 

 have been a predaceous or zoophagous Mammal. 



The long diameter of the crown of the larger of the two teeth (op. cit., taf. I, fig. 4) 

 is one eighth of an inch ; it is divided into two low subobtuse lobes or cones, each of 

 which shows tuberculate subdivisions ; the lobes are united at the exposed side by a basal 

 ridge or 'cingulum.' Each root contracts to a subobtuse closed end. This tooth is 

 preserved imbedded in the matrix. 



The second tooth (ib., fig. 3, a-d) is of smaller dimensions ; the fore-and-aft diameter 

 does not exceed one line (=1-2 th inch) ; the transverse diameter is little more than a third 

 of a line, the tooth being of compressed form, from side to side. The outer side of the 

 crown (PI. I, fig. 15, of this Monograph) presents one chief cone at the anterior half, suc- 

 ceeded by a low tubercle, and then a smaller one; the inner side of the crown (ib., fig. 14) 

 shows three or more smaller tubercles, decreasing in size as they approach the hind end of 

 the tooth, but the foremost is lower than the one on the outer side of the crown. They are 

 joined anteriorly by a small basal talon. Thus, the grinding surface of this tooth (PI. I, fig. 

 15 a) shows an irregular longitudinal mid depression between an outer and an inner tuber- 

 culate ridge or rising of the crown. It is a type of molar which prevails in a still existing 

 member of the multidentate division of the Marsupial order {Mp~mecobiusfasciatu8,y^i\\., 

 PI. I, fig. 19, from upper jaw, magn. 4 diam. ; fig. 20, from lower jaw, magn. 4 diam.), and 

 which we shall find repeated in the ultimate molar of the lower jaw of a small predaceous 

 paucidentate Marsupial from an Upper Mesozoic deposit (comp. PI. I, figs. 14 and 15, 

 with PI. IV, fig. 9 B, Plagiaulax minor, and ib., fig. 12 b, Plagiaulax Becklesii). 



The tooth (PI. I, figs. 14, 15, 15 a) from Degerloch is, most probably, a mandibular 

 molar, and, like those in Myrmecohius, is supported by an anterior longer and narrower 

 root and a posterior shorter and rather broadei" root, both tapering, with progressive 

 widening of their interval, to their implanted end. 



The crown is short or low in proportion to its fore-and-aft breadth, constituting about 

 one third the length of the entire tooth, which is about two lines. The teeth of the micro- 

 lestian type still manifested by the Australian Insectivore (PI. I, fig. 24) are quite as minute, 

 with as short crowns, as in Microlestes antiquus ; they are carried by an animal with a 



' " Naclidem audi das unter dem Namen ' Basilosaurus' bekannte, mit 2-wurzlichen Zahnen versehene 

 Fossil, mit welchem man die Stonesfielder Kiefer zu Reptilien-resten stempeln woUte, unter die Cetaceen 

 versetzt war." — Loc. cit. 



2 Gr. fitKpoi, small; \r}aTr)s, ravener ; op. cit., p. 165. 



