238 Dr. PfeifFer^s Observations on the Family Helicidae, 



ally foreign to the ovule, and introduced into it from with- 

 out. 



EXPLANATION OF PLATE IV. 



Fig, 1 . Part of the ovule and conducting tissue of Phytolacca decandra, 

 a. Conducting tissue, b. Pollen tube. c. Embryo, d. Embryo-sac. e. 

 Nucleus. /. Secundine. g. Primine. (Schleiden.) 



Fig. 2. The extremity of the pollen tube (embryo) indenting the em- 

 bryo-sac. o. Pollen tube. b. Embryo, c. Embryo-sac. (Schleiden.) 



Fig. 3. The inferior part of an ovule of Carduus nutans, after impregna- 

 tion, a. Pollen tube. b. Embryo, c. Embryo-sac. d. Nucleus, e. Te- 

 guments. (Schleiden.) 



Fig. 4. Section of the ovarium of Zea Mays at an early period of its de- 

 velopment, a. Primine. b. Secundine. c. Nucleus, d. The little cavity 

 in which the primary utricle is afterwards formed. (Mirbel and Spach.) 



Fig. 5. The same at a more advanced period, a. The primary utricle. 

 (Mirbel and Spach.) 



Fig. 6. The primary utricle, detached from the ovule, filled with the glo- 

 bulo-cellular cambium. (Mirbel and Spach.) 



Fig. 7. The embryo detached, a. Cotyledon, b. The first leaf of the 

 plumule, c. The second leaf of the plumule, (Mirbel and Spach.) 



Fig. 8. The embryo at a more advanced period, a. The first leaf of the 

 plumule, b. Radicle, c. The suspensor. (Mirbel and Spach.) 



XXVI. — Observations on the Family Helicidae, and descrip- 

 tion of a new Genus. By Dr. L. Pfeiffeb of Cassel*. 



The most difficult question concerning the limits of genera 

 among the land moUusca has of late been frequently treated 

 of, and with widely different results. If, on the one hand, 

 Ferussac went much too far, in comprising nearly all air- 

 breathing mollusca with four tentacula in his genus Helia?, still, 

 on the other hand, the attempts at a division of this large 

 group have not yet succeeded in a satisfactory manner. Dra- 

 parnaud^s genera, however, form a good basis, to which I am 

 inclined with slight deviation to return. Lamarck evidently 

 relied too much on individual peculiarities of the shell, be- 

 cause he was not acquainted with a sufficient number of spe- 

 cies in which the transitions of the forms may be distinctly 



* From Wiegmann's Archiv, Part I. 1840. 



