Miscellaneous. 465 



come the short, compressed, conical teeth figured by Sir Everard 

 Home in the 'Philosophical Transactions' for 1818-1820, where 

 they have a compressed, more or less hollow base ; but in the more 

 aged animal the bases of the teeth are soUd, squarish, very rugose, or 

 divided into short tubercles or broader lobes. 



In the Museum of the College of Surgeons there is the skull of a 

 yoimg specimen, and another of an animal rather older than the one 

 above described ; and in the British Museum there is one rather 

 older, showing the gradual change in the form of the teeth, and in- 

 termediate between the younger state and the jaws figured by Sir E. 

 Home, which are also to be seen in the College of Surgeons' Museum. 

 In the British Museum there is the skull of an aged individual, in 

 which the teeth have solid rugose and lobed bases, as above described. 



The change in form is so great that I was inclined at one time to 

 consider the skull of the young animal as forming a genus distinct 

 from Platanista, which is always characterized as having compressed 

 teeth ; and any one comparing the teeth of the old and young ani- 

 mals, without the intermediate gradations, might, at first sight, easily 

 come to the conclusion that they could scarcely pass from one form 

 to the other, as the long cylindrical front teeth of the young animal are 

 converted, in the older one, into short, conical, compressed ones, by 

 the wearing away of the tops and the alteration of the form of the 

 base. 



The sutures of the skull of this animal seem to be soon knit, for 

 they are well closed in the skulls of the young animals. 



Aquatic Hymenoptera. 



At a recent meeting of the Linnaian Society, Mr. J. Lubbock read 

 a paper on two aquatic Hymenoptera, one of which uses its wings in 

 swimming. Till now, the author stated, no aquatic Hymenoptera or 

 Orthoptera had been discovered, though the former group alone has 

 been estimated as comprising some 50,000 species, 3500 of which 

 live in Great Britain. In a basin of pond-water, on an early day in 

 August last, he had been astonished to see one of these Hymenoptera 

 {Polynema natans) quite at ease in the watery element, and actually 

 swimming by means of its wings. At first he could hardly believe 

 his eyes ; but having found several specimens, and shown them to 

 some friends, the fact was undoubted. The same phenomenon, 

 moreover, was again observed, within a week, by Mr. Duchess, of 

 Stepney. Another of the aquatic Hymenoptera, now first described 

 under the name of Walkeria aquatica, was fouud in the same pond; 

 but this, unhke the former, which swam by means of its wings, held 

 its wings motionless when under water, and used its legs only ; and 

 though these were neither flattened nor provided with any well- 

 developed fringe of setse, they seemed very well to serve this pur- 

 pose ; indeed the motion of this species was more rapid than that of 

 the former. Both species are fond of creeping along the sides of the 

 vessel in which they are kept, or on the leaves and stems of aquatic 

 plants ; but they frequently quit their support, and swim boldly out 



Ann. &; Mag. N. Hisl. Scr. 3. Vol.yd. 31 



