MUSEUM OF COMPARATIVE ZOOLOGY. 5 



and one on the symphysis. Behind the teeth proper, on each jaw there 

 is a patch of scales similar to those on the lips at the angle of the mouth. 

 The anterior row iu each of these patches, being somewhat regular, was 

 wrongly counted as teeth in the preliminary description. Backward 

 the size of the teeth decreases. There is also some change in the shape, 

 but the change from teeth with broad base, three cusps, and two but- 

 tons, to scales with a single cusp, is sudden and decided ; i. e. they do 

 not grade into each other. A strong lens, however, is necessary to dis- 

 tinguish them, since in the hinder row each cusp looks much like a single 

 scale. In the front teeth the median cusp is but little longer than the 

 others ; it curves directly backward, and does not extend much beyond 

 the prongs of the base. The lateral cusps of the same tooth incline 

 laterally and curve backward. The points are slightly bent upward. 

 Between the cusps, on the inside of each of the lateral, and on both sides 

 of the median, a slight ridge runs from the base toward the apex ; it 

 also connects with the button. On the outside of the laterals this keel 

 or ridge is obsolete, except very near the base. Striations do not appear 

 on the first rows of teeth. At the junction of cusp and base the enamel 

 is inflated or swollen into a low ridge or collar around the base of the 

 cusp ; this ridge is marked by slight prominences and hollows, as if folds 

 once existing in the enamel had disappeared, leaving only these traces- of 

 their presence. Anteriorly the cusps are greatly bent back toward the 

 base ; posteriorly they are nearly or quite erect. The base is broad and 

 long. On its upper side a ridge runs backward behind each of the but- 

 tons. These ridges end in a pair of prongs, which extend beneath the 

 base of the next tooth in the row. In front of the prongs, between their 

 bases, a small pore marks the opening of a vessel which, descending for- 

 ward, passes to the lower side to reappear in the anterior portion of the 

 tooth's base. Except at the opening of this vessel, the groove, from the 

 notch between the prongs and forward under the base, is not open as 

 figured in Plate VI. ; its covering, however, is translucent, very thin, 

 and easily carried away. On each side of the ridge in which this groove 

 lies there is a concavity for the reception of the basal prongs of the pre- 

 ceding tooth. Outside of each of these indentations there is a rounded 

 prominence which is situated beneath the base of a lateral cusp. About 

 a third of the length of the base of each tooth extends under that of the 

 next behind it in its row. 



Backward the characters of the teeth change. In the sixth and 

 seventh rows the little prominences around the base of a cusp have 

 become shallow plications or foldings in the enamel, which iu the ninth 



