520 Zoological Society. 



across the base. It consists of a body of compact dentine composed 

 of successive lamelliform cones, and has a short and wide conical 

 cavity at the base. 



From the teeth supposed to have belonged to the Poikilopleuron, 

 the specimens above described differ in the ridges on the crown 

 being greater in number and more closely set, as well as in the form 

 of the teeth being nearly circular instead of elliptical ; from the teeth 

 of the Pliosaurus they differ also in being round and not three-sided, 

 and in having longitudinal ridges over the whole surface of the crown ; 

 and from the teeth of the Mosasaurus they differ in being ridged 

 and not smooth. 



In conclusion, Mr. Owen states, that as the Hythe Saurian is di- 

 stinct from all other described Saurians, and as these teeth belonged 

 to a great Saurian also undescribed, and further, as the Maidstone 

 tooth was found in the same formation as the Hythe fossil, so it may 

 be convenient to consider all these remains for the present to have 

 belonged to the genus Polyptychodon, originally proposed for the 

 animal which was pro\'ided with the teeth, 



ZOOLOGICAL SOCIETY. 



April 13, 1 841.— William Yarrell, Esq., Vice-President, in the Chair. 



The following paper, on some new genera of Birds, by Hugh E. 

 Strickland, Esq., was read. 



" It is not without some unwillingness that I venture to point 

 out some new generic groups of birds, because I am of opinion that 

 the process of naming and defining new genera has been in many 

 cases carried too far already. The class of birds probably does not 

 contain more than 6000 species, and these have already been dis- 

 tributed into upwards of 1000 genera, and I think therefore there 

 can be no doubt that systematic ornithologists are now fast approach- 

 ing the point beyond which it will not be expedient to carry the 

 subdivision of the older groups into new genera. For we must not 

 lose sight of the fact, that expediency or practical convenience does 

 form an element in the construction even of a natural system. In 

 such a system the materials which constitute any group must be na- 

 turally allied, and they must be placed in such oi-der as will best 

 show their natural affinities, and yet the number of nominal genera 

 into which such a group is to be divided may be a question of mere 

 expediency. For the species, which are the only real ingredients in 

 a family or subfamily, often pass from one form of structure to an- 

 other without any hiatus, so that it becomes a mere matter of opinion 

 whether the so-called genera into which they are to be classed shall 

 be many or few. Nature draws no line by which the rank or ex- 

 tent of genera can be determined. As a general rule, varieties of 

 form are considered to constitute genera, and varieties of colour, 

 species ; but this criterion is far from infallible, for we very rarely 

 find two species, however closely allied, possessing precisely the 

 same form and proportions ; so that if every difference of structure 

 be held to be generic, we shall end in having as many genera as 



