from the Marl- Slate of Midderidge. 59 



being turned down to oppose those of the upper jaw. At the 

 posterior part of the general mass several of the second primary 

 and petalodontoid teeth lie scattered about, chiefly with the 

 under surfaces uppermost. 



The remaining specimen (PI, 11. fig. 1) to be noticed, though 

 consisting of only a few teeth, is very interesting, inasmuch as 

 it displays in profile an entire vertical row, lying in almost 

 exact order, one resting upon the other. The whole length of 

 the teeth is seen, from the cutting-margin to and including 

 the root, bent in a deep sigmoidal curve. The series appears 

 to be of the central teeth : four lie in close contact, the back or 

 under surface of one individual resting upon and fitting exactly 

 to the face or upper surface of that immediately below it, A 

 very imjierfect fragment of a fifth tooth is seen pressed to the 

 under surface of the fourth of the series ; and in front consi- 

 derable portions of two lateral primaries lie with their under 

 surfaces uppermost, one of which exhibits in a remarkable 

 manner the transverse grooves caused by the rubbing of the 

 crushing-disk of the tooth on that supporting it. Similar 

 transverse grooves can be seen on one or two other teeth of the 

 series. A considerable fragment of a second primary lies near 

 the centre of the row. 



The minute structure of the teeth is rather peculiar ; and 

 though we have not examined it in the entire tooth, and though 

 om* account of it must necessarily be imperfect, as it is from 

 mere fragments, yet we cannot refrain from saying something 

 on the subject. 



We have stated that the upper surface (namely, the anterior 

 scoop-like margin and the crushing-disk) is covered with a 

 layer of opaque-white enamel-like matter. This coating is 

 thickest over the crushing-disk, where it is of a considerable 

 depth. When the tooth is quite fresh, there appears to overlie 

 this a thin film of transparent enamel. The interior is com- 

 posed of a rich brown-coloured substance, which may be looked 

 upon as a form of dentine, made up of large, branched and 

 anastomosing tubes with thick walls, which, for the most part, 

 run lengthwise ; their cavities are undoubtedly medullary chan- 

 nels ; they are narrow in proportion to the thickness of the en- 

 tire tube. These give ofl", almost at right angles, small, irre- 

 gular, branched and anastomosing tubes, which, penetrating 

 the overlying white enamel-like matter, abut near to the sur- 

 face. The white matter also penetrates occasionally into the 

 interior of the tooth, insinuating itself between the tubes ; but 

 the central portion is usually so exceedingly dense that few 

 traces of it are to be observed. On tlie iq)per or concave face 

 of the cutting-margin, however, the dentinal tubes, which are 



