1866. YOUNG PLATYS0MU8. 307 



the microscope^ , however, transverse slices of the jaws show that 

 the two ridges are made up of dentine, and are connected by a thin 

 layer of the same substance which floors the intervening groove. 

 The whole plate thus constituted is lodged in an alveolus, whose thin 

 margins overlap the base of the dental plate, covering the pseudo- 

 enamel or cement layer which invests that portion of the plate. 

 The dentine is very finely granular, and traversed by medullary 

 canals, which are most divergent in the centre of the tubercles. These 

 canals bifurcate once, seldom twice, and never anastomose. The 

 branches reach the surface of the dentine, and there terminate in 

 funnel-shaped pits, similar to those by which they open into the 

 pulp-cavity. Each canal is surrounded by an areola of darker colour 

 than the intervening clear dentinal space. This areola is narrow in- 

 feriorly, but its area soon exceeds that of the canal, and maintains 

 this proportion to the surface. The areola is made of tubes which 

 come off at right angles to the canals, form a fine network, and, at 

 the margin of the areola, give off finer tubes, which traverse the den- 

 tine in an obliquely upward direction, anastomosing, without inter- 

 vening dilatations, with the similar tubes from adjacent canals, so 

 as to form a network, whose interspaces are diamond-shaped. In 

 consequence of this upward direction of these fine tubes, a triangular 

 space is left between the bases of the canals, which is traversed by 

 vertical tubes springing directly from the pulp-cavity. The canals of 

 the dentine which floors the median groove are only half the diameter 

 of those in the tubercles : they are close-set, and connected either 

 directly, or by the intervention of branches which pass for the 

 most part at a low angle from the canals. The areola is narrow : 

 the tubules of the clear space are larger than in the tubercles, and 

 form a less angular network. These differences in the proportions 

 of the parts, differences rather of degree than kind of structure, 

 commence abruptly on either side of a vertical line which may be 

 drawn from the point where the tubercle begins to rise above the 

 plane of the groove. At the outer inferior angle of the base of the 

 dental plate, but not encroaching on the surface which is turned 

 towards the substance of the jaw, a layer of bony tissue commences 

 as a thin investment, which rapidly thickens upwards, and appears 

 above the alveolar margin, it also occupies a part of the surface of 

 the median groove. Its further extension towards the apex of the 

 tubercles, or its replacement by a thin enamel layer, of which only 

 fragments survive, cannot be ascertained; the extreme brittleness 

 of the specimens prepared for the microscope having rendered the 

 process of grinding unusually difiicult. The pseudo-enamel, or 

 cement, is divisible into three lamellse, a clearer separating two dark 

 layers ; all these are more granular than the dentine. The under 

 surface is projected into conical processes, which fit into the terminal 

 infundibula of the medullary canals. The tubes given off from these 

 canals enter the deepest layer of the cement in which they form a 

 network of equal-sized tubes : this network invades part of the clear 



* The details of this interesting dentitional type are reserved for fuller de- 

 scription and illustration. 



