GEOWTH OF THE TOOTH OE ECHINUS. 
393 
the enlarged extremities of the flabelliform processes the skii'tings, and the enamel rods 
the white coating on the outer surface of the dorsum of the body of the tooth. These 
data being recollected, the particular direction and arrangement of the tubular inter- 
spaces, indeed the whole course and character of the histology of the mature structure 
which I am about to describe, become easily intelligible. 
Histology . — The ultimate structm-e of the mature tooth of the Echinus is remarkable 
as presenting appearances similar to the hard structures of Vertebrata and certain Mol- 
lusca — hone, dentine, and shell, and exhibiting also a curious inconsistency in the aspects 
which it displays in different lines of section, vertical or transverse. The vertical section 
from before backwards resembles very closely the bone of fishes, while the vertical 
section of the tooth Horn side to side presents appearances closely simulating the dentine 
of a mammalian tooth. A transverse section exhibits a large area of the structure 
exceedingly like an oblique section of some molluscous shells, such as Pinna, with circum- 
scribed regions of minute parallel tubes very like mammal ivory. Further, some por- 
tions of the tooth, the “ skirtings,” closely resemble the shell of echinoderms generally. 
A vertical section from within outwards of the compact consolidated portion of an 
EcMnus-tooth, when viewed by moderate magnifying power, exhibits a general laminated 
sti’ucture, the laminations being parallel, and running in two dmections from the axis 
of the tooth obliquely downwards and outwards, and downwards and inwards (Plate VI. 
figs. 2 & 3) ; these laminEe are commonly straight in their course. Professor Williamson 
speaks of the latter, those of the keel, as ha\ing a double sigmoid course. This, hownver, 
appears to vary with the species : I have not met with it in the teeth of the small variety 
of E. s])h(era, among which I have mostly prosecuted my researches, while I have found 
it very conspicuous in the large teeth of E. Fleiningii. Interspersed among the laminae, 
but stiQ more between them, are minute opake spaces (Plate VIII. fig. 7), which with 
higher pow'ers are seen to resemble very closely the lacunae of true vertebrate bone, 
especially that of fishes, in the angularity of the lacunae and the sharp bends of the 
canahculi (Plate VIII. fig. 8). The central portion or axis of the tooth is seen to be 
veiy transparent, presenting an indistinct and latent lamination, but destitute of opake 
vacuities. The difficulty of reconciling the appearances of the transverse and vertical 
sections (Plate VII. fig. 5, a, h, and Plate VIII. figs. 7 & 8) depends partly on the form 
and partly on the arrangement of the interspaces in the tissue, especially as regards the 
bone-hke lacunee. Now the lacunse have a certain length, about of an inch, 
and a breadth of about half that measui’ement ; but they are so compressed laterally, 
that when seen cut across as in a transverse section they present but a minute vacuity, 
scarcely more than the canaliculi themselves. In arrangement the lacunse are so dis- 
posed in the tooth that their long axis is not far from vertical — in the keel obliquely 
from above downwards and inwards ; in the body of the tooth from above downwards 
and outwards. Their extreme breadth is at right angles to their length, and from before 
backwards in the tooth. Thus it is in a transverse section the lacunae are seen at their 
minimum, whereas in a vertical section (from before backwards, not from side to side) 
