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PROFESSOR WYVILLE THOMSON ON THE ECHINOIDEA OE THE 
reous styles are scattered in the walls of the vessel. The ovaries are botryoidal, loosely 
lobed, and of rather small size. The ovarial ducts are very wide, and the ova are 
remarkably large. No calcareous framework could be detected in the investing mem- 
brane of the ovaries, but small irregular fenestrated plates are imbedded in the wall of 
the intestine. 
With the general facies of Cidaris, Porocidaris purpurata differs in many remarkable 
details. Probably the character of the highest physiological import is the structure and 
great size of the ovarial openings, particularly when taken in connexion with the unusual 
size of the ova. The form of the pedicellarise and the form and arrangement of the 
scales of the buccal membrane are likewise highly characteristic. 
Four specimens were taken with the dredge at one station about 100 miles to the 
north of the Hebrides. One or two spines of apparently the same species were dredged 
by Mr. Gwyn Jeffreys off Cape Espichel. As I have already said, I have no doubt that 
this species must be associated, at all events generically, with the tertiary species of 
Porocidaris , and that it is one of the many examples, which are now forcing themselves 
upon our notice daily, of the persistence of tertiary, and even of older, types in deep water 
to the present day. 
No representatives of the Saleniadse or or the Diadematidse were procured in these 
dredgings. 
II. 
Received April 4, 1873. 
Family 2. Echinothurida:, fam. nov. 
Regular Echinoideans with the plates of the test imbricated and the test flexible. 
The plates of the interambulacral areas overlap from the mouth towards the apex, and 
the ambulacral plates in the opposite direction. The plates of the ambulacral areas are 
within those of the interambulacral areas, and their ends are overlapped by the ends of 
the adjacent interambulacral plates. The peristome is covered by ten double rows of 
imbricated calcareous scales, and five alternate double rows of these are perforated for 
the passage of tube feet in continuous series with those of the ambulacral areas of 
the corona, as in the Cidaridse. The edge of the peristome is entire, as in the Cidaridae. 
The spines are hollow, as in the Diadematidse. The ambulacral tubes on the oral surface 
of the test are provided with a sucking-disk supported by a rosette of fenestrated calca- 
reous plates, while those on the apical surface are long, conical, and pointed, without 
either sucking-disks or terminal pits. On both aspects of the body the walls of the 
tubes contain fenestrated calcareous spicules. 
The dental pyramid is large and strong. Epiphyses are developed on the upper and 
outer angles of the pairs of tooth-sockets, but there is no arch formed. The teeth are 
deeply and symmetrically channelled within. The auriculae are large and strong, forming 
closed arches, and their bases are united across the interambulacral spaces by strong 
