North American Helices. 37 



Maxilla costis numerosis, parum prominentibus, raargine crenato. Fol- 

 liculi mucosi, sagitta, capreolus, flagellum desunt. 



15. Polygyra Say. 



T. umbilicata vel perforata, orbiculato-planata, oblique costulato-stri- 

 ata ; anfr. 5-7^ lente accrescentes, ultimus antice constrictus, breviter 

 deflexus, basi inflatus, devians, penultimus plane conspicuus, perfora- 

 tioaem ssepissime riraatim constringens ; apertura subreniformis vel irre- 

 gulariter sinuata ; perist. anguste reflexum, callosum, marginibua inter- 

 dam dentatis, callo triangulari, deutiformi, ia parietem aperturialem 

 oblique intrante junctis. 



16. Stenotrema* Raf. 



T. obtecte perforata, lenticularis vel globoso-depressa, pilosula ; anfr. 

 4^—6, ultimus antice gibbus, breviter deflexus, basi tumidus ; spira 

 parum elevata; perist. albo-Iabiatum, raargine supero breviter reflexo, 

 bas;di strictiusculo, saepe sinuoso dentate 



In quarta parte circuitus anfractus ultimi ante aperturam, columella 

 appendice callosa, lamelliformi, cavationem anfractus coarclaute, rnunita 

 est. 



17. Triodopsisj Raf. 



T. obtecte perforata vel umbilicata, orbiculato-depressa vel subglobosa, 

 plus minus oblique striata; anfr. 5-7, ultimus antice paululum de- 

 flexus; apertura sinuoso-coarctata, subtriangularis ; perist. albo-callo- 

 sum, late angulatim reflexum ; paries apcrturialis dente valido, oblique 

 intrante munitus. Maxilla costis 3-5, prominentibus, margine dentato. 

 Folliculi mucosi 2, simplices. (Moq.-Tand.) Sagitta 1 subconica, parum 

 arcuata, basi biangulata, elongata. (Ad. Schmidt.) 



18. MesodonJ Raf. 



T. umbilicata vel obtecte perforata, subglobosa vel orbiculato-depressa, 

 tenuis, subtiliter striata, interdum decussatim sculpta ; anfr. 5-6 regulares ; 

 apertura rotundato-lunaris, interdum dente parvulo in pariete aperturiali 



* " Chimotrema. — Aperture transverse, extremely curved, resembling a simple 

 fissure. 



"Toxotrema. — Differs from the preceding Ly the emarginate lip. 



" Stenotrema. — Differing from the two preceding by a thick emarginate lip, and 

 a second lip flattened to the spire ajic 1 uniting with the true lip; a transversal 

 carina above." — Raf. Vide Terr. Moll. I. c. 49. 



\ "Triodopsis. — Umbilicus large, lip thick, aperture narrowed by three teeth, 

 one upon each lip and one upon the columella." Raf. A r ide Terr. Moll. I. c. 49. 



% " Mesodon. — Differs from Helix by lower lip with a tooth." Raf. Vide Terr. 

 Moll. I. c. 49. 



