726 
PEOFESSOE WYVILLE THOMSON ON THE ECHINOIDEA OF THE 
localities, I am quite satisfied that no characters of specific value distinguish the two 
forms. The one passes through every intermediate gradation into the other, and fre- 
quently a single locality yields a complete series, including characteristic examples of 
both extremes. 
2. Cidaris affinis , Philippi. (Plate LX.) 
Principal synonyms: — Cidaris affinis, Philippi, Wieg. Archiv, vol. i. 1845; Cidaris 
Stokesii, Agassiz, Cat. raisonnee, 1846 ; Cidaris affinis , Sars, Middelhavets Litt.-Fauna, 
1857; Leiocidaris affinis, Dujardin and Hupe, 1862; Cidaris (Dor ocidaris) jpapillata, 
A. Agassiz, Revision of the Echini, 1872. 
The test of a full-grown specimen of this species is about 25 millims. in diameter. 
It is slightly more depressed than in C. papillata. The general arrangement of the 
plates of the test is the same, but the ambulacra are somewhat wider in proportion to 
the width of the ambulacral area?. The pore-arese are also relatively somewhat wider. 
There are nine or ten pairs of pores opposite the edge of one of the large in ter ambulacral 
plates of the ambitus. The interambulacral plates have usually two rows of large-sized 
tubercles surrounding the areola, so that, instead of there being only a single row of the 
larger flattened spines converging over the areola, there is a double series of such spines. 
The radioles are long and rather delicate, ornamented as in C. 'paffillata, only the pointed 
tubercles are much more pronounced. The spines from the larger areolae taper to a fine 
point : those round the mouth are shorter, stouter, and more cylindrical ; and imme- 
diately round the mouth some of them are slightly flattened. The general colour of the 
animal is a very brilliant cinnabar shaded with dark brown. The large spines are usually 
banded cinnabar and brown. 
This is a very beautiful and well-marked little urchin, but I admit that its claim to 
specific rank is doubtful. Some varieties approach very closely to stunted, shallow-water 
forms of C. paffillata, and particularly that special to the Mediterranean ; and many 
(some of them very vivid in colour) were dredged in Tangier Bay. 
Genus Porocidaris, Desor. 
The ambulacra are narrow and strap-shaped, and slightly sinuous. The ambulacral 
arese are narrow and bear miliary granules only. The pore-arese are simple, the pores 
of each pair closely approximated. The interambulacral plates are large, eight to ten 
in a row. The areolse are large and elliptical, and show a tendency to become confluent. 
The bosses of the primary tubercles are smooth or crenulated. The tubercles are of 
moderate size, smooth and perforated. The ovarial plates are large, halbert-shaped. 
A deep arch-like cleft passes inwards through about one third of the width of the plate, 
and a corresponding cleft separates through part of their length the two last interambu- 
lacral plates of each double series, so that the two clefts together form a large diamond- 
shaped opening through the test. This opening is filled up by a strong chitinous mem- 
brane, and a large pore penetrates the centre of the membrane, affording an exit to the 
duct of the ovary. The primary radioles are large and long, cylindrical or pointed round 
