140 EIGHTH REPORT 1838. 



ivory commence. The superficies of the tooth is slightly punctated, 

 but the depressions do not correspond with the mouths of tubes, but 

 with the interspaces of whole groups of the coarser tubes. 



Psammodus. — A transverse section of the tooth of this genus pre- 

 sents the appearance, under a moderate magnifying power, as if it were 

 composed of close-set coarse tubes, the arese of which were thus ex- 

 posed. Such a section, viewed with a power of 400 diameters, shows 

 that these tubes are surrounded by concenti-ic lamellas, exactly as the 

 Haversian canals ; and that these lamellae, and the clear interspace, 

 which is generally equal to the thickness of the lamellaj, are permeated 

 by minute irregularly disposed tubes, which anastomose in the clear 

 interspace, and open into extremely minute cells, scattered in the same 

 part. A longitudinal section of the same tooth shows the whole course 

 of the canals ; they run nearly perpendicularly to the convex super- 

 ficies of the tooth, and, consequently, incline outwards at the sides of 

 the section. They lie nearly parallel with each other, M'ith interspaces 

 equal to from 6 to 8 times their own diameter, and branch dichoto- 

 mously once or twice in their course. Each canal is surrounded by 

 concentric layers of a dark colour, encroaching upon one-third of the 

 interspace, which thus presents tMO dark streaks and one intermediate 

 right line : the whole of these interspaces is perforated by the irregular 

 wavy, branched, anastomosing calcigerous tubes. The terminations of 

 the canals near the periphery of tiie tooth are slightly dilated, and give 

 oif in every direction calcigerous tubes corresponding to those in the 

 interspace of the canals. The structure of the tooth of Psammodus 

 differs from that of Acrodus in the greater number and more parallel 

 course of the canals, their fewer branches, and want of anastomoses, 

 and in the absence of a distinct external enamel-like layer of very fine 

 tubes. 



Ptychodus latissimus. — The structure of this tooth has a close affinity 

 to that of Psammodifs : it is composed of Haversian canals and cal- 

 cigerous tubes proceeding therefrom. The base of the tooth is com- 

 posed of close- set and irregular canals, and is very opaque : the canals 

 emerge from this part half-way to the grinding surface, to Avhich they 

 proceed perpendicularly. They difler from those of the Psammodus 

 in being wider, more close-set, and more branched, — the branches 

 being given off" at more open angles, and the terminal ones being larger 

 in jiroportion to the trunks. The papillose surface of the tooth is com- 

 posed of the terminations of the inoxtiicably interwoven fine calci- 

 gerous tubes given oft' from the terminations of the canals. The inter- 

 spaces of the canals are also occupied by the same minute anastomosing 

 reticulate tube-work. Numerous minute calcigerous cells are also 

 present in the interspaces. There is a clear substance coating the 

 grinding surface of the tooth, in which neither tubes nor any definite 

 structure could be detected, though, from analogy, such doubtless 

 exist. The darker substance, forming the concentric lamellae around 

 the canals, occupies the same proportion of their interspace as in the 

 Psam?nodus. 



Chimcera.--Tihe tooth of this fish appears, when a section of it is 



