224 Note on Irish Fresh Water Shells. 



of L. octanfracta, given to me by Mr. G. B. Sowerby. The 

 specimen of Planorbis spirorbis is large, and is so strongly 

 marked in form with reference to the type, PI. vortex, which 

 I took in ditches near the Royal Canal, Dublin, and in 

 Lough Cavrigan, county of Cavan, that I cannot agree with 

 Mr. Thompson in his dereliction of the example of the 

 English and Continental authors, who consider the species 

 to be distinct. 



1 Planorbis spirorbis, Miiller. 



Hab. Ditches by the road side on the lands of Tracy's 

 town near Hillburne, west of Taghmon, county of 

 Wexford. 

 2 Lymnsea glabra, Mull. 



octanfracta, Mont. 



leucostoma, Lam. 



elongata, Drap. 



Hab. with the last. The road in question leads from the 

 gate of Hillburne in the direction of the sea. From the 

 short space of time that I could spare to collect my speci- 

 mens, I am of opinion, that the species must be abundant 

 in the locality. 



At Bannow, in the same county, Helix aspersa was abun- 

 dant in banks on the cliffs about the sea ; and in a clear 

 burn falling down a ravine of the Forth mountain at Newbay, 

 I got specimens of Aneylus flavintilis adhering to stones. 



W. H. Benson. 



Note on the Self-calculating Sextant, proposed by Captain 

 Jack. — By Captain Campbell, Assistant Surveyor Ge- 

 neral, Madras Establishment 



I do not perfectly comprehend the plan of the instrument 

 proposed by Captain Jack, (vol. i. page 521,) and am at a 

 loss to guess at the mode he proposed for laying down on the 

 face of the instrument a scale of co-tangents, the co-tangent 

 of a small arc being of an infinite length, an extent rather 



