206 PEOCEEDINGS OP THE ACADEMY OF 



fore and aft measurement, and the height is usually little less than the latter. 

 The root partakes of the form of the outline of the base of the crown, but is 

 more square and is flat or transversely concave below. 



Twelve specimens of teeth of Ptychodus Mortoni, from the Cretaceous For- 

 mation of Alabama, belonging to the Yale College Museum, have been sub- 

 mitted to my examination by Mr. William M. Gabb. Among them occurs 

 the largest tooth of the species I have seen, and larger than any on record. 

 It is labelled as having been derived from Perry Co., Alabama. The fang 

 and parts of the lateral and back borders of the crown are broken away. In 

 the perfect condition the crown has measured a little over two inches in 

 transverse diameter, one inch and a quarter antero-posteriorly, and ten lines 

 in height. The crushing surface is porportionately less prominent at the 

 centre than in the smaller teeth attributed to the same species, and is more 

 uniformly convex, or less expanded laterally at the base. The borders of the 

 posterior sinus also are less abrupt or defined. The unworn summit presents 

 a crucial ridge, of which the lateral radii are most distinct and directed pos- 

 tero-laterally. From the crucial ridge, numerous ridges, equally prominent, 

 diverge, branch in their course and ultimately conjoin in a fine reticulation at 

 the base af the crown. This reticulation has the greatest breadth at the sides 

 of the crown and is least developed at the fore-part. 



Eleven teeth from Uniontown, Alabama, exhibit a gradation in size from less 

 than three-fourths that of the above described specimen down to one little 

 more than a fourth of its diameter. The specimens present a remarkable sim- 

 ilitude throughout. Some are proportionately wider fore and aft than others, 

 and the smallest are porportionately higher than the largest ones. The outline 

 of the base of the crown is reniform, with the relation of the longer and 

 shorter diameters varying. The largest specimen has the crown an inch and 

 a half wide, a little over three-fourths of an inch fore and aft, and about half 

 an inch in height. The sides of the crown expand at the base laterally ; the 

 fore-part forms nearly a uniform slope, and the back surface slopes to the 

 sinus, which forms a broad triangular depression. The fang is fourteen lines 

 wide, seven lines fore and aft, and three lines in depth. 



The crown of a median sized tooth of the series, unworn, measures scant 14 

 lines wide, 7^ fore and aft, and 65 high. The smallest specimen has the crown 

 7 lines wide. 4J fore and aft, and an equal height. Its base laterally appears 

 more abruptly expanded than in the others. Most of the specimens are unworn 

 and e.xhibit the characteristic ridges of the crown in a striking manner. In 

 three specimens the coarser ridges are resolved into the reticulation much 

 earlier or nearer the summit than in the others. In one specimen the crown 

 is smooth or totally devoid of ridges, presenting the same appearance repre- 

 sented in figs. 4, 5, pi. XXX, of Dixon's Geology of Sussex, and described as 

 " nascent or incomplete teeth of Ptychodus. 



Seven specimens of teeth in the Museum of the Academy, from Alabama, 

 exhibit the same characters expressed in the description of those above. They 

 all present an unmistakeable specific likeness, though varying in the propor- 

 tions of their diameter. The largest specimen has the croAvn 16 lines wide, 11 

 lines fore and aft, and 8 lines high. The root is an inch wide, 7 lines fore and 

 aft and nearly 3 lines thick. A second specimen, with the crown 16 lines wide 

 and 9 lines fore and aft, has been proportionately lower than the former. Its 

 summit is worn away, leaving an exposed circular disk of vaso-dentine 4 lines 

 in diameter. 



Two specimens in the Museum of the Academy, presented by Prof. Joseph 

 Jones, are from Green Co., Alabama. The larger is perfect and unworn. The 

 crown is scant 14 lines wide, by 7 lines fore and aft, and 5 lines high. The 

 root is 11^- lines wide, 4J fore and aft, and 2 lines thick. 



Two specimens in the Museum of the Academy, presented by Dr. Wm. Spill- 

 man, are from Columbus, Mississippi. They present the same character as the 

 Alabama specimens. The larger specimen has the crown 20 lines wide, 10 lines 



■ [Sept. 



