278 QUARTERLY CHRONICLE. 



the common stock of scientific knowledge^ or how much (if 

 any) is at actual variance with well ascertained facts. One of 

 the most important of the generalisations alluded to is the 

 division of the class mammalia in regard to the times of 

 formation and the succession of their teeth, into two groups ; 

 the Monophyodonts, or those that generate a single set of 

 teeth, and the Diphyodonts, or those that generate two sets 

 of teeth ; the Monophyodonts including the orders Monotre- 

 raata, Edentata, and Cetacea, all the rest of the class being 

 Diphyodonts. The teeth of the former group are more simple 

 and uniform in character, not distinctly divisible into sets to 

 which the terms incisor, canine, premolar, and molar, have been 

 applied, and follow no numerical law. The group is, in fact, 

 equivalent to that which the term Homodont has been applied 

 by some authors. On the other hand, in the Mammalian orders 

 with two sets of teeth, these organs are said to acquire fixed 

 individual characters, to receive special denominations, and 

 •can be determined from species to species, being equivalent 

 to the Heterodonts. The author then showed that among 

 the Homodonts the nine-handed Armadillo was certainly a 

 Diphyodont, having two complete sets of teeth, and among 

 the Hetorodonts many were partially, and probably some 

 comjiletely, Monophyodonts. Moreover, that almost every 

 intermediate condition between complete Diphyodont and 

 simple Monophyodont dentition existed, citing especially 

 the Sirenia, Elephants, Rodents, and Marsupials. He then, 

 by the aid of diagrams, showed particularly two modes of 

 transition between monophyodont and diphyodont dentition — 

 one in which the number of teeth changed was reduced to a 

 single one on each side of each jaw, as in marsupials, and 

 the other in which the first set of teeth, retaining their full 

 number, were reduced to mere functionless rudiments, and 

 even disappearing before birth, as in the case of the seals, 

 especially the great elephant seal. These observations showed 

 that the terms '^^monophyodont'' and "diphyodont," though 

 useful additions to our language as a means of indicating 

 briefly certain physiological conditions, have not, as applied 

 to the mammalian class, precisely the same significance that 

 their author originally attributed to them. The classification 

 and special homologies of the teeth of the heterodont mammals 

 was next discussed. Certain generalisations as to the pre- 

 vailing number of each kind of teeth in different groups of 

 - animals were sustained, but deviations were shown from some 

 of the rules laid down — such as that when the premolars fall 

 short of the typical number, the absent ones are from the 

 fore-part of the scries. The general infcrciiee was that. 



