22 



young H. bursatella, only the whorls remain rounded instead of 

 being flattened. 



Helix obolus. T. parva, discoidea, deorsum compressa, su- 

 pra planulata, infra concava, radiatim minutissime et inequaliter 

 striata, fusco-ferruginea, piceo tessellata : spira anfr. 5^ supra 

 excavatis, ultimo superne costato-carinato : apertura sub-rhom- 

 boidea ; fauce lamella unica secundum anfractum penultimum 

 volvente. Lat. -i, alt. 5^ poll. Hah. Taheiti. 



A most singular discoidal shell, resembling Planorbis vortex. 



Helix bursatella. Testa parva, per-variabilis, rotundata, 

 conica vel planulata, supra maculis fuscis et albidis alternanti- 

 bus fiexuosis, tessellata, infra cinereo-albida vel flammulis fuscis 

 flexuosis radiata, costis numerosissimis acutis radiantibus lyrata : 

 subtus convexiuscula, interdum simplex, interdum concentrice 

 striata, nunc radiatim costellata, nunc albida, nunc fusco flammu- 

 lata : spira anfr. ad 8 externe excavatis, ultimo carinato ; sutura 

 impressa, plerumque marginata : apertura parva, transversa ; 

 fauce cum lamellis 7 volventibus, quorum 2 superiores, 1 colum- 

 ellaris, 4 remotis (1 supra, 3 infra carinam) ad labium ; labro 

 simplici ; umbilico cavernoso, in junioribus acetabuliformi, in 

 majoribus ad januam parvo, constricto, margine acuto, una cum 

 labio continuo. Hab. Taheiti and Eimeo. 



The above characters are found combined in every possible 

 manner. The variations in size, color, solidity, and umbilicus, 

 may all be reasonably ascribed to differences in age, food, and per- 

 haps to the elevation, between 2000 and 5000 feet, at which they 

 were collected. The armature of the mouth and the peculiar 

 umbilicus, at first broad, and at maturity contracted, or pursed 

 up, so that the flattened edges of the whorls form a large spheri- 

 cal cavity, are the characteristic marks. Two principal varieties 

 may be noticed. 



". Large, elevated, conical specimens, which are without mot- 

 tling, rib-like strice, or revolving lines beneath. Lat. -fa, alt. fa 

 poll. 



§. Smaller, depressed specimens, more or less coarsely ribbed, 

 striated and mottled, both above and beneath. Lat. ^, alt. fa 

 poll. 



Helix tumulus. Testa parva, solida, flavida, orbiculato-pyra- 

 midata, basi sub-planulata, umbilico modico perforata : spira 



