219 
panied by the elimination of the primary tubercle on alternate 
plates, as is carried to an extreme in such forms as Tripneustes 
and Holopneustes, or by the adoption of a polyporous condition; cf. 
Ingolf-Echinoidea I. p. 132—133) — the demonstration ot the fact 
that in the tuberculation the main point is not the number of 
tubercles on an interambulacral plate but the distance between the 
tubercles, — and pass to another most important part of the work, 
viz. that dealing with the masticatory organs. 
The manner in which Jackson treats these rather intricate 
anatomical structures is so clear and lucid as to deserve sincere 
admiration"). Though these structures have been the object of 
study several times, and even by so eminent an investigator- as 
Lovén, Jackson has succeded in finding a character (— pits 
in the dorsal side of the half-pyramids, below the epiphyses —) 
which, together with such other features as the structure uf the 
teeth and the shape of the foramen magnum, is of primary value 
for classification. On the basis of these characters he establishes 
the three suborders of the ''Centrechinoidea”: Aulodonta, Stirodonta 
and Camarodonta, being thus characterized: The Aulodonta have 
grooved teeth and narrow epiphyses, not meeting in suture over 
1) When speaking of the inner projections from the ambulacral plates 
in Hyattechinus and the Cidarids (p. 61) Jackson states that A. 
Agassiz has described such ””spines extending into the body from 
the inner face of the peristomal ambulacral plates of Porocidaris 
Cobosi, but I believe they have not been noticed before in coronal am- - 
bulacral plates”. I may mention that Wyv. Thomson (""Porcupine” 
Echinoidea 1872. p. 728) gave a description and very good figure of 
these structures in Porocidaris purpurata, as pointed out by Agassiz 
in the place quoted by Jackson. Further Ch. Stewart in his 
paper "On certain Organs of the Cidaridæ” (Trans. Linn. Soc. 2. Ser. 
Zool. I. 1877), quoted by Jackson himself, gives a pair of excellent 
fignres (Pl. 70, 6—7) of these ambulacral processes in Cidaris tribu- 
loides and Phyllacanthus baculosa, and in his description henne 
(p. 571) he refers to the descriptions and figurés of such ”fvertebral pro- 
Cesses” given already by Joh. Miller. For the sake of completeness 
I may remark that in the Ingolf-Echinoidea (I) I have mentioned the 
existence of such ambulaecral processes in Cidaris affinis, Stereocidaris 
ingolfiana and rare papillata, giving figures of those in the 
two latter (p. 40, Pl. VI. 5—6). It is thus not a quite new fact 
which Jackson has here irer 
