‘PORCUPINE’ DEEP-SEA DREDGING-EXPEDITIONS. 
731 
calcareous ridges. The auriculae have the effect of springing from the edges of the 
ambulacral plates ; but this character seems to have but little value, as the ring of cal- 
cified elements forming the auriculae and their uniting ridges appears to be entirely 
distinct, merely forming adhesions with the ambulacral and interambulacral elements 
of the perisom in its course. 
The peristome and the periproct are unusually large ; the peristome entire in outline, 
and the peristomial membrane mailed, as in the Cidaridae ; and the periproct somewhat 
irregular in form, owing to the large size of the ovarial plates. The pores of the ambu- 
lacral areas are trigeminal ; but the two pairs of each arc nearest the central line of the 
area are approximate to one another, and each pair penetrates an accessory plate, the 
two accessory pore-plates being intercalated between two of the ambulacral plates, while 
the third pair of pores of the arc is remote from the others, and penetrates the substance 
of the ambulacral plate near its outer edge. The ambulacral areas are rather wide, 
usually more than one half the width of the interambulacral areas. 
The Echinothuridse occupy an intermediate position between the Cidaridse and the 
Diadematidse, certain very important anatomical characters associating them with the 
former family, while in general facies and habit they more nearly resemble the latter. 
As in Cidaris the edge of the peristomial opening is entire, and the oral branchiae are 
absent ; the peristomial membrane is covered with imbricated calcareous scales, with 
their free edges studded with tubercles for spines and pedicellarise, and perforated for 
lines of tube-feet which carry the ambulacra up to the edge of the mouth. As in 
Cidaris , Diadema , and Echinocidaris , the tube feet on the apical surface of the test are 
conical, with neither sucking-disks nor terminal pits ; and as in Cidaris, Diadema , and 
Echinocidaris, the epiphyses of the tooth-sockets do not unite into an arch over the 
roots of the teeth. As in Cidaris and Diadema, there is no central ridge dividing the 
inner tooth-groove, and, as in many species of the Diadematidse, the spines are hollow. 
From the Cidaridse, the Echinothuridse differ in the arrangement and in the delicacy 
of the calcareous structures of the perisom, in the relations of the auriculse (which start 
from the edges of the ambulacral areas, and not from the interambulacral plates), and 
in the structure of the spines. From the Diadematidse, they differ in the absence of 
external oral branchise and branchial notches, and in the continuity of the ranges of 
pores up to the edge of the mouth ; and they differ from the Echinidse in the absence 
of longitudinal ridges dividing the concavity of the teeth. 
The flexibility of the shell, the continuity of the ambulacra over the membrane of the 
peristome, the hollow spines, the peculiar arrangement of the pairs of pores, the singu- 
larly large ovarial openings, the absence of calcified arches uniting the elements of the 
tooth-sockets, the simply grooved teeth, the duck-billed pedicellarise, and the singular 
arrangement of the walls of fenestrated fascia bounding the ambulacral Spaces, form 
together an assemblage of characters which altogether preclude the fusion of the group 
with any family hitherto defined. It is very possible that Asthenosoma varium, Grube 
(45 ter Jahres-Bericht d. schles. Gesell. f. vat. Cult. Breslau, 1868), may belong to this 
