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XX. On the Stnicture and Groietli of the Tooth of Echinus. By S. James A. Salter, 
M.B. Bond., FL.S., F.G.S. Communicated hy Thomas Bell, F.B.S. 
Eeceived Mareli 5 , — Eeacl March 21, 1S61. 
The researches upon which this memoir is based were prosecuted more than four years 
since, and the illustrations which accompany it were mostly executed at nearly as 
distant a date — before I was 9 ,ware that this interesting and obscure subject had ever 
been investigated by competent observers. And though it appears that some of my 
own observations (independent as they were) have been anticipated, there is so much of 
this ground of inquiry untrodden, and the accounts published of the structures in 
question are so very imperfect and often incorrect, that I have been induced to put toge- 
ther in this paper the results of my own more extended investigations. 
The literature of this subject is confined to very narrow limits; for though compara- 
tive anatomists, from the time of Aristotle, have not failed to describe the curious 
apparatus of teeth and jaws in the Echinus, I am not aware that any observer had inves- 
tigated the structure and growth of the teeth themselves, or published any account of 
their intrinsic anatomy, before the Essay on the genus Echinus was written by Valentin 
as one portion of a general monograph on the Echinodermata published by Agassiz 
and which appeared in 1841. 
The illustrations of Valentin’s paper are many of them exceedingly good, as far as 
they refer to the anatomy of the Echinus-tooth ; but this distinguished anatomist signally 
failed to interpret correctly the appearances he has figured, and to associate in the mature 
tooth-structure the elementary parts with the position they there occupy. 
Valentin has figured correctly and fairly described the earliest growth of the plumule 
of the tooth (tab. G, figs. 113 and 114) with its two rows of triangular plates; but he 
subsequently speaks of the confluent apices of these plates as constituting the keel of the 
matured tooth, thus inverting their du’ection of growth and placmg them in a portion of 
the tooth which they never approach (figs. 115 and 116), and this too while he figures 
most accurately, and in an admirable illustration, the relationship of the enamel rods, 
the keel fibres and plates ; but ^dewing the latter in vertical section and not knowing 
the pre\dous position of the plates and plan of their arrangement, he mistakes their line- 
like section for fibres, and so designates them (see fig. 110). 
Valentin gives no correct account or figures of the ultimate histology of the Echinus- 
tooth ; but by some accident he represents as a portion of a tooth a very highly magni- 
* Anatomie des Ecbinodermes. Premiere Monographie : Anatomie du Genre par G. Yalektix. 
Neucliatel, 1841. 
MDCCCLXI. 3 H 
