28 



a Britisli species, being proba])ly imported in tlie larva state in 

 foreign timber, and, so far as is known, not multiplying here. 

 We find in the instrument from -which the saw fly derives its 

 name, one of the most beautiful of all the contrivances that 

 have been observed for the placing of the eggs of insects, an 

 instrument from which (if the chronologj'^ of arts and sciences 

 would aUow us to believe that optics had ever been in advance 

 of mechanics) we might suppose that man had borrowed 

 not the idea only, but the perfect pattern of the saw fly. This 

 instrument is a very curious object, and in order to describe it, 

 it will be proper to compare it with the tenon saw used by 

 cabinet makers, which, being made of a very thin plate of steel, 

 is fitted with a back to prevent its bending. The back is a piece 

 of iron, in which a narrow and deep groove is cut to receive 

 the plate, which is fixed. The saw of the Tenthredo is also 

 furnished with a back, but the groove is in the plate, and 

 receives a prominent ridge on the back, which is not fixed, but 

 permits the saw to slide forward or backward as it is thrown 

 out or retracted. The saw of artificers is single, but that of the 

 Tenthredo is double and consists of two distinct saws with their 

 backs. The insect, in using them, first throws out one, and, 

 whUe it is returning, pushes forward the other ; and this alter- 

 nate motion is continued till the incision is effected, when the 

 two saws, receding from each other, conduct the egg between 

 them into its place. In the artificial saw the teeth are alternately 

 bent towards the sides, or out of the right line, in order that 

 the fissiu'e or skerfmaj be made suflieiently wide for the blade 

 to move easily. To answer this purpose in some measure, in 

 that of the Tenthredo the teeth are a little twisted, so as to stand 

 obliquely with respect to the right line, and their point of 

 course projects a little beyond the place of the blade without 

 being laterally bent, and all those in each blade thus project 

 a little outward. But the skerf is more effectually made and a 

 free range procured for the saws, by small teeth placed on the 

 outer side of each, so that whUe their vertical effect is that of a 

 saw, their lateral effect is that of a rasp. In the artificial saw 

 the teeth all point outwards (towards the end) and are simple, 

 but in the saw of the Tenthredo they point inwards or towards 

 the handle, and their outer edge is beset with smaller teeth 

 which point outwards (towards the end). 



Captain McDakin drew the attention of the Society to a 

 Septaria from the Weald Clay of the Isle of Wight, 

 containing fifty-two per cent, of sulphate of barytes. In 

 comparing this with a septaria from the London Clay, 

 also containing barj-tes and exhibited on a previous occa- 

 sion, attention was directed to the difference of the two foi-ma- 

 tions, the London Clay being a marine deposit while the Weald 



