‘ PORCUPINE ’ DEEP-SEA DREDGING-EXPEDITIONS. 
727 
the ambitus, but in the neighbourhood of the peristome becoming flattened, curved, 
longitudinally striated, and strongly toothed round the edge ; in fact assuming a very 
peculiar and characteristic form. The pedicellarise are numerous and two-valved, a 
structure as yet unique, so far as I am aware, among Echinoideans. The auricles are 
fused together through the greater part of their height in the middle of the interambu- 
lacral areae, sending off short wing-like processes which arch over towards the ambulacra, 
but scarcely extend beyond their edges. The peristomial opening, the buccal membrane, 
and the dental pyramid and teeth have much the same character as in Cidaris. 
I feel very little hesitation in referring the following very remarkable form to the 
genus Porocidaris of Desor, even although it is wanting in the very character upon which 
that genus was founded, and although the characters upon which its claims to generic 
distinctness are based are not shown in any of the fragments which alone have been met 
with in a fossil state. M. Desor gives the following characters of the genus. The 
areolae, which have the form and general character of those of Cidaris , are pierced 
round the edge by a series of apertures ; these pores are placed in the course of little 
grooves which radiate from the boss. The alveoli have a tendency to become confluent. 
The tubercles are perforated and crenulated. The radioles are compressed into the 
form of plates, and are curved, deeply grooved longitudinally, and strongly toothed along 
the edges. From their very remarkable form several species have been founded upon 
these radioles alone. 
Some separate plates have been found associated with the radioles of one species only, 
Porocidaris verronensis , Schmidel, from the Nummulitic Tertiaries of Verona and of the 
neighbourhood of Nice ; the other species (P. serrata, D’Archiac, from beds of the same 
age near Biarritz ; P. serraria , Broun, from the Miocenes of Castel Arguato ; and P. 
Schmidelii , Munster, from the Lower Oolite of the valley of the Erick) rest upon the 
evidence of the spines only. The last species M. Desor quotes with some hesitation, as 
it would be somewhat remarkable to meet with the same type at such distant periods, 
while they do not seem to occur in intervening beds. The recent species dredged off 
the north of Scotland has the spines, the perforated and crenulated bosses, and the 
coalescing areolae of Porocidaris, which genus is known to come down as far as the 
Miocene Tertiaries ; and it must be remarked that many animal forms hitherto known 
only as Miocene fossils are met with living at great depths in the present sea. 
M. Desor attached much significance to the ring of perforations in the. scrobicular 
area. In this I cannot concur. Several species of Cidaris, and even more markedly the 
species of Porocidaris under discussion, have radiating muscular impressions for the 
insertion of the muscles which move the large spines. It is perfectly conceivable that 
these depressions may sometimes actually go through the test, or at all events leave it 
so thin that the action of water and attrition during the process of fossilization may be 
well supposed to have completed the perforation. 
5 F 
MDCCCLXXJV. 
