219 
pamed by the elimiuation 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 faet 
that in the tuberculation the main point is not the number of 
eicles on an inteiambulacial plate but the distance between the 
tubercles, and pass to another most important part of the work, 
viz. tbat dealing with the masticatory organs. 
The manner in which Jack son treats these rather intricate 
anatomical structures is so clear and lucid as to deserve sincere 
admiration 1 ). Though these structures have been the object of 
stud} several times, and even by so eminent an investigator as 
Loven, Jack s on has succeded in tinding a character (— pits 
in the dorsal side of the half-pvramids, below the epiphyses _) 
which, together with such other features as the structure of 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”: Aulodonia, Siirodonta 
and Camarodonta , being thus characterized: The Aulodonta have 
grooved teeth and narrow epiphyses, not meeting in suture over 
) When speakmg of the inner projections trom the ambulacral plates 
in Hyattechmus and the Cidarids (p. 61) Jackson states that A. 
Ag assi z 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 Agas si z 
m 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 
ngures (PI. 70, 6-7) of these ambulacral processes in Cidaris tribu- 
loides and Phyllacanthus baculosa, and in his description thereof 
(p. 571) he refers to the descriptions and figures of such “vertebral pro¬ 
cesses” given already by Joh. Muller. Por the sake of completeness 
I may remark that in the Ingolf-Echinoidea (I) I have mentioned the 
existence of such ambulacral processes in Cidaris affinis, Stereocidaris 
uigol iana and Dorocidaris papillata, giving figures of those in the 
two latter (p. 40, PI. VI. 5-6). lt is thus not a quite new faet 
wnicn Jacks o li has liere produced. 
