1883.] NATURAL SCIENCES OF PHILADELPHIA. 225 



very large cylindrical cells (PI. X, fig. 5, i, ^, <5, 4 and 5), to 

 which I have given the name of matrix. When a horizontal 

 section is examined these cells are very striking and easily dis- 

 tinguished b}'^ their having a much lighter color than the sur- 

 rounding parts ; each one of these five cells has a peculiar and 

 characteristic form. The cell marked 1 stands obliquely to the 

 tunica, and that end farthest from the tunica is rounded or dome- 

 shaped ; all the other of these five cells, with the exception of 4i 

 are pointed at the corresponding extremity, and also placed 

 obliquely' to the tunica. In 4 tliis condition is reversed, the 

 pointed extremity being nearest to, but not touching, the tunica. 

 The blunt end of this cell is in contact with the radula, and the 

 point is inserted between cells 3 and 5. 



The protoplasm of these five cells of the matrix is quite clear, 

 taking only a slight reddish tinge with borax (Grenadier's) car- 

 mine. There is not the slightest trace of a granulated structure 

 to be found. The nuclei of these cells are very large and oval in 

 form; their size is about twice that of the nuclei that are found 

 in the neighboring tongue-papilla (PI. X, fig. 5 m). The structure 

 of these nuclei diff"ers somewhat among themselves ; some contain 

 only one nucleolus, in others it is more or less broken up, and 

 others still have a granular appearance. 



The cells 1, 2 and 8 form the basal membrane (PI. X, B. 31.) 

 and cell 4 the bases of the teeth. The convex end of cell 1 

 secretes a mass of conchyolin, which is the beginning of the basal 

 membrane. The posterior part of this membrane, narael}^ that 

 part which lies against cell 1 in the figure (fig. 5), has the appear- 

 ance of a hook, the point of which lies between cells 1 and 5, just 

 overlapping the tip of the point of 3. These three cells are those 

 which take part in the formation of the basal membrane of the 

 radula, the cell 3 forming the upper, and cell 1 the lower face of 

 this so-called hook, and cell 2 probably adds a little to the point. 

 This hook-like appearance is only present in longitudinal sections. 

 In%reality, naturally, this part of the basal membrane is not a 

 hook, but a sharp edge, which is curled over and fits into a groove 

 formed by two rows of cells ; cells like cell 1 (fig. 5) forming the 

 anterior, and cells like cell 2 forming the posterior wall. 



The formation of the teeth is cari'ied on by the cell marked 4- 

 This is triaiigular in shape with the base abutting the posterior 

 face of the tooth, d (PI. X, fig. 5). I believe that this cell 4- is 



