Royal Society, 59 



the requisite constancy of character in a large proportion of the 

 class Mammalia. Certain members of this class, e.g. the order 

 Bruta and the Cetacea vera, have teeth too numerous and alike in 

 form and mode of development to admit of being determined indi- 

 vidually from species to species. Such mammalia have but one set 

 of teeth, and the author proposes to call them 'Monophyodonts.' 

 On the other hand, the orders Marsupialia, InSectivora, Rodentia, 

 Ruminantia, Pachydermata, Carnivora, Cheiroptera, Quadrumana 

 and Bimana have two sets of teeth, and might be called collectively, 

 ' Diphyodonts.' Of the permanent teeth of this division of mam- 

 malia, some succeed the deciduous teeth vertically, others come into 

 place behind one another in horizontal succession. The 'incisors' 

 are determined by a character of relative position to the jaws and 

 to each other: so likewise the ' canines.' The remaining teeth are 

 divided into those which are developed in vertical relation to the 

 deciduous molars, and push them out, and those that have not such 

 relation, but follow each other horizontally : the term ' molar ' is re- 

 stricted by the author to these latter teeth, and that of ' premolar ' 

 to the former ones, which are always anterior to the molars. There 

 is a remarkable degree of constancy in the number of these different 

 kinds of teeth ; in the placental Diphyodonts, e. g. the ' incisors ' 



3 — 3 

 never exceed , i. e. 3 on each side of both jaws, the * ca- 



1 \ 4 4, 3 3 



nines' _ ■ > the premolars , _^ , the molars =44; and this 



the author regards as the typical formula of dentition in the great 

 proportion of the mammalian class above denned. It was rarely 

 departed from by the primaeval species that have become extinct, 

 and is modified chiefly by defect or loss of certain teeth in the ex- 

 isting species. When the grinders are below the typical number, 

 the missing molars are taken from the back part of their series, and 

 the premolars from the fore part of theirs : the most constant teeth 

 being the fourth premolar and first true molar ; these are always 

 determinable, whatever be their form, by the relation to them of the 

 last tooth of the deciduous series. Thus determined, the homo- 

 logies of the other grinders are ascertained by counting the molars 

 from the first backwards, 1, 2, 3 ; and the premolars from the last 

 forwards, 4, 3, 2, 1. The symbols are made by adding the initial m 

 to the numbers of the molar teeth, and the initial j» to those of the 

 premolar teeth. The author concludes by pointing out the advan- 

 tages of this system of anatomical notation. 



2. " Description of the Hydrostatic Log." By the Rev. E. L. 

 Berthon, M.A. Communicated by Sir Francis Beaufort, F.R.S. 

 &c, on the part of the Lords Commissioners of the Admiralty. 



The object of this invention is to obtain a register of the speed 

 of ships, by a column of mercury, in such a manner that the height 

 of the column shall depend upon the velocity alone, and not be 

 affected by any disturbing causes, such as alteration of draught of 

 water, pitching and rolling, &c. 



The principle embraces that of Pitot's tube, inasmuch as the 



