Quarterly Journal of Conchology. 379 



In conclusion, Dr. Jeffreys expresses his inability to refer a 

 single species of cretaceous moUusk to recent forms, and depre- 

 cates the modern theory of evolution as unsupported by facts, 

 which appear rather to favor devolution or succession. 



To all conchologists and geologists we would recommend a 

 careful perusal of this deeply interesting and important address, 

 containing, as it does, the carefully digested and tabulated results 

 of some of the most important scientific expeditions of modern 

 times. J. W. T. 



DESCRIPTION OF A NEW SPECIES OF PLANORBIS. 

 By W. nelson. 



Planorbis (Gyraulus) Gibbonsi (Ph IV., f. 3). 



Shell depressed and somewhat concave in the middle, above 

 and below ; rather thin, of a light horn color, closely and finely 

 striate in the line of growth, epidermis rather thin, periphery 

 rounded, luJiorls 3 Js, somewhat rapidly enlarging, the last flattened 

 below, as broad or broadei than the rest of the shell and deflected 

 near the aperture, suture deep and distinct, mouth broadly ellip- 

 tical, the two ends equally rounded, /.jm/t?///^ thin and continuous, 

 umbilicus very shallow. 



Diam. 0.175; ^^t. 0.045. 



Hab. " On decaying vegetation in a stagnant pool, Zanzi- 

 bar." (Gibbons.) 



This very distinct species is nearly allied to Planorbis glaber, 

 Jeff., but may be easily distinguished from it by the flattened 

 whorls. It has also some affinity, judging by the description and 

 figure, with Planorbis Nat ale ns is, Krauss. 



Mature specimens are thickly incrusted. 



This Planorbis was discovered by Mr. J. S. Gibbons, to whom 

 I have much pleasure in dedicating it. 

 [uly, 1878. 



