Echinoid Tests , Cidaroida,. 
67 
Regularia Endobranchiata, in vvhich the peristome is central; the periproct is cen¬ 
tral on the aboral surface of the body, and is surrounded by the apical System of 
plates. The ambulacra each consist of two vertical series of simple narrow plates, 
some of vvhich may be demi-plates. The interambulacral plates are unituberculate, 
bearing large spines. There is a dental apparatus». [See footnote, p. 65!] 
Dr. Gregory placed the genera Tiarechinus and Lyseclünus in distinct Fami- 
lies, laying stress on the greater number of interambulacrals in the latter genus, 
and especially on the limitation of its ambulacra to the oral surface. Accepting 
Dr. Gregory’s account of those interambulacrals, which he himself admits to be 
doubtful, it will be observed that the arrangement in Tiarechinus can, as he says, 
«easily have been produced from it by the resorption of the second zone of inter¬ 
ambulacral plates and increase in height of those of the third zone», or, it may be 
added, by the vertical fusion of those in the third and fourth zones. Tiarechinus 
princeps may therefore in its youthful stages have had interambulacrals like those 
thought by Gregory to exist in Lysechinus. Quite apart then from the extreme 
uncertainty attaching to it, the character does not appear of more than generic 
importance. But by describing the ambulacra of Tiarechinus as «desmactinic» 
(1900, p. 305) and those of Lysechinus as «lysactinic» (1897, p. 1004), Dr. Gregory 
Postulates a fundamental difference between the two genera. This difference simply 
does not exist. «Desmactinic» means that the ambulacra are continuous from peri¬ 
stome to apical System, as in Echinoidea generally; while «lysactinic» means, not 
only that they are limited to the oral surface, but that they are separated from the 
apical System by intervening plates as in Asteroidea. But in Lysechinus, as Gregory’s 
own diagram shows, the ambulacra bear to the apical System precisely the same 
relation that they bear in Tiarechinus. The oculars of Lysechinus are represented 
as far larger and as passing down between the interambulacrals, but not as beijig 
separated from the ambulacra. Therefore the ambulacra are not lysactinic. They 
are not even «limited to grooves on the oral surface», for, as shown in Gregory’s 
figures 1 b and 2b, their aboral portions are clearly visible in side view. Lambert 
(1900), then, did well in reducing the Families Tiarechinidae and Lysechinidae to 
the rank of Sub-Families, but von Zittel (1903) and Delage and Herouard (1904) 
did better in retaining only the Family Tiarechinidae. The Statement contained in 
the Family-diagnosis of the latter authors that «une des plaques interambulacraires 
empiete sur la membrane peristomienne», would, if correct, ally the Family to the 
Archaeocidaridae rather than to the Lepidocentridae. But it is due to some misap- 
prehension, since the fact that the interambulacrum begins with a single plate, which 
does not invade the peristomial membrane, is one of the most important characters 
of the Family. The following is a 
Revised Diagnosis of the Family Tiarechinidae. 
Cidaroida with plates closely united into a small rigid test, of which nearly 
half is occupied by the apical System. Ambulacra short, with plates all primaries 
and pore-pairs uniserial. Interambulacra begin with a single peristomial plate, and 
increase gradually or immediately to three plates in a horizontal row. 
Revised diagnosis of Tiarechinus. 
A Tiarechinid in which the single peristomial plate of each interambulacrum is 
succeeded by a horizontal row of three vertically elongate plates, the outer ones 
abutting on the ambulacra, the oculars, and the genitals. 
5* 
