ANATOMY OF ACLESIA FREERI. 77 



the base. Each layer consists of prismatic columns, which are 

 superposed, or coincide, from layer to layer. Each plate is situ- 

 ated on an epithelial bed with slightly elevated margins, the shape 

 of which corresponds to the shape of the bottom of the plate. 

 An area is visible in the center which corresponds to the base of 

 the axial column of the plate. 



The smaller plates of the anterior row are more slender, and 

 pointed, and are considerably curved toward the interior part 

 of the gizzard (fig. 19, Plate II). It is noticeable that the 

 small stomach-plates are all found on the anterior edge of the 

 gizzard, while the largest plates are found in the posterior row. 

 Among the anterior plates, also, and located on the extreme 

 anterior edge of the gizzard, often may be found minute plates 

 in the first stages of growth. This arrangement suggests that 

 possibly new teeth are continually being produced at the anterior 

 edge of the gizzard, which gradually move backward as they 

 grow, room being made for them by the reabsorption, or falling 

 away, of the most posterior plates. Such a process would be 

 essentially similar to the forward movement of the bands along 

 which the nautilus is attached to its shell as its body moves 

 forward in the growing shell. 



The axial column is the first part of the plate to be formed, 

 growing from a minute, subtriangular, flat, epithelial elevation 

 at the anterior edge of the gizzard. The outer coating is then 

 formed around this, almost entirely, at first, upon the posterior 

 side, where it frequently develops in a second point. The axial 

 column is the more sharply pointed of the two; its tip is not 

 covered by the secondary deposit until the plate is of consider- 

 able size. A second axial column develops just in front of the 

 first, fused with the first at its base, but with freely projecting 

 tip. With further growth of the plate, the axial column now 

 develops as a single structure, while the double point becomes 

 worn away. The outer substance is formed in considerable 

 thickness upon the anterior surface of the axial column, while 

 it remains as a thin layer upon the sides. As a result of this 

 method of deposition, the axial column usually extends almost 

 completely across the plate and forms the entire grinding sur- 

 face, and the outer substance forms considerable masses anterior 

 and posterior to the axial column (figs. 17 and 19, Plate III). 

 The surfaces of the axial column are iridescent. 



The elongated conical stomach, which follows the gizzard, 

 shows no definite demarcation from the intestine. It is about 

 35 millimeters in length, and curves to the right under the 



