S94 
ME. S. J, A. SALTEE ON THE STEIJCTHEE AND 
they are displayed at their maximum. In the body of the tooth, where this vertical 
section does not absolutely correspond in parallelism with the disposition of the ele- 
mentary parts, as it does in the keel, the lacunae are not shown in their longest axis 
nor in their extreme breadth — they are displayed somewhat obliquely, and are propor- 
tionally reduced in size. 
The canaliculi run in greatest numbers and in general direction from the sides of the 
lacunne more than from their extremities — at right angles, that is, to the general distri- 
bution of the lacunae, at right angles to their long axis, and to the general fibrous or 
lamelliform structure of the particular part in which they are observed. This is espe- 
cially obvious in vertical sections of the keel (Plate VIII. figs. 7 & 8), and it heightens 
the semblance of the tissue to true vertebrate bone — the fibres and plates lookmg like 
bone-laminae of the Haversian systems, or of the external compact tissue, the lacunae 
being dispersed mostly between the laminae, while the canaliculi run outwards and in- 
wards to the nutritional surfaces ; and doubtless the anatomical arrangement of the 
tubular canals and reservoir cavities is, in both instances (vertebrate bone and Echinus- 
tooth), equally in accordance with their functional office 
The dentine-like structure of the Echinus-tooth, as seen in transverse section, is con= 
fined to the lateral regions of the body of the tooth — the portion formed by the plates, 
and the flabelliform processes between the plates, the tubes being the parallel intervals 
which are left by the imperfect adhesion of these contiguous elements. That this is 
absolutely the case is easily shown by examining a rather thick section, when, by alter- 
ing the focus of the microscope, the successive tubes, one above another, are recognized 
and are traced nearer to, or further from, the edge of the tooth, in accordance 'with the 
oblique direction in which the plates are disposed. The tube-like inteiwals remind the 
observer closely of the dentinal tubes of a human tooth, and the cavities have about the 
same diameter. There are moreover many fine branchings like what are seen especially 
towards the peripheral extremity of the true dentinal tubes of a mammalian tooth. 
Very similar appearances are to be observed in the same region of the body of the 
tooth, that is within the outer edge and not quite in the centre, in a vertical section from 
side to side — not from before backwards. In the absolute centre there is a lateral 
parallel linear arrangement, but there are no tubular intervals. 
Formmg the outer limit of the dorsal surface of the body of the tooth is a structure 
to which I have applied the term enamel, and this rather from its whiteness and its 
anatomical position than from its histological character. This is the only matured 
structure of the Echinus-tooth which presents precisely the same aspect, whether seen 
in vertical or transverse section. The enamel is a very thin layer, only reacliing the 
3 ^th or 4 ^ 0 ^^ of i’^oh in thickness even in large teeth, and that too in its thickest 
part — the lateral extremities and the dorsal convexities of the body of the tooth (see 
Plate VI. fig. 4, c, c). 
The arrangement of this structure is a series of cylindrical tubes running from within 
outwards in an otherwise apparently homogeneous mass of carbonate of lime. Theh 
