March 20, 1890] 



NATURE 



467 



an outer cusp ; while by the suppression of one of the 

 primitive cusps we arrive at the quadritubercular tooth, 

 bunodont tooth (Fig. C), hke that of the Pigs. In the 

 upper molars the primitive triangle in what is termed the 

 secodont series may remain purely tricuspid. But by 

 the development of intermediate tubercles in both the 

 secodont and bunodont series a quinquetubercular form 

 is reached ; while the addition of a postero-internal cusp 

 in the bunodont series gives us the sextubercular molar. 



There is no doubtas to the homologyof the three primary 

 cusps in the upper and lower molars ; and Prof. Osborn 

 proposes the following series of terms for all the cusps 

 above mentioned. The first secondary cusps (hypocone 

 and hypoconid) respectively added to the upper and lower 

 molars are also evidently homologous, and modify the 

 crown from a triangular to a quadrangular form ; but 

 there is no homology between the additional secondary 

 cusps of the upper molars termed protoconule and meta- 

 conule with the one termed ectoconid in the lower molars. 



Protocone 

 Hypocone 

 Paracone 

 Metacone 

 Protoconule' — ml 

 Metaconule — pi. 



•pr. 

 hy. 

 pa, 

 •me. 



Terms applied to the cusps of molars : 



Upper Molars. 



Antero-internal cusp 



Postero- ,, ,, or 6th cusp 



Antero-extemal ,, . 



Postero- ,, »> 



Anterior intermediate cusp 



Posterior ,, ,, 



Lower Molars. 

 Antero-external cusp 

 Postero- ,. i» 

 Antero-internal or 5th cusp 

 Intermediate, or antero-internal cusp 



(in quadritubercular molars) 

 Postero-internal cusp 



Having thus worked out the homology and relations of 

 the tooth-cusps. Prof. Osborn gives some interesting 

 observations on the principles governing the development 



= Protoconid — pr"* 

 = Hypoconid — hy'' 



Paraconid 



Metaconid 

 Entoconid 



-pa'' 



^ 

 (^ 



Fig. B. — Upper and Lower Molars in mutual apposition, i, Delphimis ; 2, Droiiiatherium ; 3, Tricoyf.iion; 4, Peralcstcs and Spalacotheriiim ; 



Si Didyinictis ; €, Mioclaniis ; 7, Hyopsodits. Letters as in preceding figure. 



of these cusps. It is considered that in the earliest 

 Mammalian, or sub-mammalian, type of dentition (Haplo- 

 dont), the simple cones of the upper and lower jaws 



-'Yvl 



ml 



fir 'hf 



Fig. C;. — Diagram of two upper and lower quadritubercular molars in appo- 

 sition The cusps and ridges of the upper molars are in double lines, 

 and those of the lower ones in black. The letters refer to the table given 

 above. The lower molars are looked at from below, as if transparent. 



interlocked with one another, as in the modern Dolphins 

 (Fig. B, i). The first additions to the primitive protoconid 



appeared upon its anterior and posterior borders, and the 

 growth of the para- and metaconids involved the 

 necessity of the upper teeth biting on the outer side of 

 the lower (Fig. B, 2), this condition being termed anisogna- 

 thism, in contrast to the isognathism of the simple inter- 

 locking cones. In the typical tritubercular type (Fig, A, 

 7) it has been suggested that the para- and metaconids 

 were rotated inwards from the anterior and posterior 

 borders of the triconodont type ; but it is quite possible 

 that they may have been originally developed in their 

 present position. By the alternation of the primitive 

 triangle in the upper and lower jaws of the tritubercular 

 type, the retention of an isognathous arrangement is 

 permitted, the upper and lower teeth biting directly 

 against one another. 



Finally, Fig. C shows the mutual relations of the upper 

 and lower teeth of the complicated quadritubercular 

 molars, with the positions held by the primitive tri- 

 tubercular triangles. 



OXFORD "PASS" GEOMETRY. 



ajeciifierpTriTos fxr)Sfls iuravdui elalrw. 



WHETHER poultry are to be regarded as descended 

 from a primeval egg or a primeval hen, is a 

 question on which some amount of scholastic ingenuity 

 is supposed to have been exercised, and whether teachers 

 or examiners are responsible for defective trainine in 

 geometry is a question on which much might, more or 

 less unprofitably, be said, and on which teachers and 

 examiners may be expected to take different views. 

 Happily for the mental equipment of the present genera- 

 tion of students, many teachers and examiners, avoiding 

 barren controversy, have both laboured, as far as in them 

 lies, to encourage soundness and thoroughness. 



Probably, the old-world teachers who, hearing a 

 " Euclid " lesson with the open Simson in their hand, 

 looked upon " therefore " as an unwarrantable substitute 

 J or " wherefore," and could not be induced to accept 



" angle CAB " as a legitimate equivalent for what they 

 saw in the text presented as " angle BAC," are fast dis- 

 appearing, if not already extinct. Unfortunately, we are 

 still under the influence of bad examination papers. 

 Take, for instance, the papers set last year at Respon- 

 sions. The sole directions from the examiner to the 

 printer, necessary for getting these set up, might have 

 been, and very likely were, as follows : — 



' The symbols >«/. and//, should properly apply respectively to themeta- 

 conule and protoconule, but since they bear the opposite .signification in 

 Fig. C, they are placed as above. 



