eye soon learns to detect, as by instinet, the family likeness of each species. In the seulptured ones 
there is of course no diffieulty, and the rigid observance of one form as the type is not absolutely 
necessary; but in the smooth species, determinable as they are principally by shape and outline, and 
all of them more or less minute, it is essential to have a fixed type and to adhere to it. And if 
the species themselves are diffieult to identify, the figures even in the standard works do not help 
the student much, from the diffieulty of determining what are the type forms. Three figures of the 
same species in three different works may be found to be quite unlike each other, though no doubt 
the authors had the same species before them. Sometimes, it may be, the artist has not caught 
the same aspect of the shell as the writer, and this itself might result in a considerable divergence 
from the type form. Only in this way can I account for some authors figures differing so much 
from their written deseriptions,“ 
Verzeichniss der Arten. 
A. Ptychostomon =. str. 
a. Gruppe des Pt. conoideum. 
1. conoideum, Brocchi. T. conspicuum, Alder. 13. novegradense, Brus. 
2. politum, Bivona. 8. turritum, Hanley. 14. turgidum, Sars. 
3. tenue, Jeffr. 9. turrieulatum, Mtrs. 15. umbilicare, Malm. 
4. acutum, Jeffr. 10. lukisi, Jeffr. 16. albellum, Loven. 
5. erassum, Jeftr. 11. omphaloössum, Wats. 17. nitens, Jeffr. 
6. unidentatum, Mtg. 12. pallidum, Mtg. 18. dolioliforme Jeffr. 
b. Gruppe des Pt. obliguum. 
19. obliguum, Alder. 21. truncatulum, Jeftr. 
20. scandens, Mitrs. 22. plicatum, Mtg. 
ec. Gruppe des Pt. rissoide. 
23. rissoide, Hanley. 25. fallax, Mtrs. 27. bulimulus, Brugn. 
24. nardoi, Brus. 26. erjavecianum, Brus. 28. michaelis, Brugn. 
B. Ondina, de Folin. 
29. insculptum, Mtg. 34. monterosatoi, Bucq. 38. ewilissimum, Brus. 
30. diaphanum, Jeffr. 35. warreni, T’homps. 39. marginatum, Caill. 
31. erystallinum, Mtrs. 36. fusulus, Mtrs. 40. teresianum, Arad. & Ben. 
32. elegans, Mtrs. 57. coaretatum, Sars. 41. minimum, Jeffr. 
33. nivosum, Mtg. 
