248 MARTIN F. WOODWARD. 



these being- unpaired occupies the centre of the row, and on 

 either side of this are situated 111 teeth. So far as I can see 

 from the examination of many rows this number is quite con- 

 stant. The lateral teeth exhibit a number of diiferent types, 

 at least five, which, however, merge imperceptibly into one 

 another. For convenience^ sake we may, however, follow 

 Fischer and Bouvier, and divide them into the following 

 groups: — (1) The central teeth, (2) the lamellate teeth, (3) 

 the hooked teeth or uncini, (4) the brush or tufted teeth, (5) 

 flabelliform teeth. 



The Rhachian or Unpaired Tooth.— This tooth (figs. 32 

 and 33) has a very carious form. Viewed from above (fig. 32), 

 it appears to consist of a somewhat pointed oval or lanceolate 

 lamella, which overlaps the adjacent central teeth. When, 

 however, the rhachian tooth is isolated and viewed from the 

 side (fig. 33), it is seen that this more or less horizontally 

 placed lamella is attached to a longitudinally placed vertical 

 plate, the posterior half of which is thickened, and forms the 

 base of attachment of the tooth to the basal membrane. The 

 tooth thus consists of two pieces — a flat horizontal lamella, 

 and a vertical plate strengthening and attaching the former 

 to the I'adular membrane. 



The Central Pairs. — On either side of the rhachian are 

 situated three large teeth (fig. 32), which, while asymmetrical 

 in form, nevertheless approximate somewhat in structure to 

 the symmetrical rhachian tooth, forming a gradation between 

 this tooth and the more laterally placed lamellate teeth. It 

 is very difficult, if not impossible, to draw a line between 

 these central pairs and the lamellate teeth, and we only separate 

 them for the convenience of description. In the central 

 teeth (figs. 34 — 36) the vertical plate has greatly increased 

 in size, while the horizontal lamella, so characteristic of the 

 rhachidian, is much reduced, and only pi^esent on the outer 

 side of the vertical plate near the base of attachment ; it still, 

 however, overlaps the tooth immediately external to it (fig. 

 32). The portion corresponding to the vertical plate of the 

 rhachian is now no longer placed vertically, but has become 



