708 Harorn HEATH, 
free outher surface which is homogeneous. The cells forming the 
basement membrane while not so large as in some of the pulmonates 
are nevertheless more conspicuous than in the prosobranchs and 
chitons. As Fig. 17 shows each cell is more or less triangular in 
cross section and the distal extremity is indented where it comes 
in contact with the basement membrane. The enamel cells (Zell- 
haufen of RösstLEer) occupy the bottom of the radula sac and are 
in direct contact with the epithelium on the dorsal side (oberes 
Epithel) and ventrally with the odontoblasts with which they 
gradually blend. These enamel cells are rectangular in outline but 
anteriorly become more globular or polyhedral and extending forward 
between the odontoblasts surround the teeth until the latter become 
exposed in the pharyngeal cavity. They appear to form a well 
defined and regular sheath one or two cells deep about the greater 
portion of each tooth, especially the great cusp, which more dorsally 
comes into intimate contact with the „oberes Epithel“. The material 
secreted by these cells was supposed by Rossier to form an enamel- 
like substance about the newly developed tooth and in the present 
case this material is distinctly visible as a superficial coat that 
stains darkly with haematoxylin. 
I have not made an exhaustive study of the formation of the 
radula but the evidence so far as it goes supports the observations 
of Rösster. Concerning the length of the secretory activity of 
the matrix cells there is little to be said conclusively. It is 
difficult to explain the definite relations of the enamel cells to 
the teeth on any other assumption than that they are continually 
proliferated; but whether the odontoblasts and matrix cells of the 
basement membrane are likewise renewed I am not at this time 
prepared to say. 
Immediately behind the salivary glands the character of the 
alimentary tract changes abruptly. A cross section of this region 
shows that the dorsal wall is much folded longitudinally and consists 
of high slender cells containing an abundance of some secretion 
which reacts essentially as that of the buccal tube. Laterally the 
cells of this character change to low cubical elements which form 
a fold at the sides of the radular supports and are also continuous 
at this point with the epithelial lining of the radular tube. More 
posteriorly the oesophageal canal narrows, its ventral wall also 
becomes thrown into folds (Fig. 5) and in this condition it passes 
backward and unites with the stomach. 
