REPORT ON THE ASTEROIDEA. 127 
B. With a simplified form of cribriform organ on the margins of each pair of 
marginal plates. Actinal interradial areas traversed by fimbriated 
transverse channels, in continuation of the fasciolar or cribriform 
channels between the marginal plates. : 5 . . . CTENODISCINE. 
a, A single genus : i : ‘ : : : : : ; : . Ctenodiscus. 
Another genus is ranked amongst the Porcellanasteridee by M. Perrier’ in a recent 
notice of the starfishes dredged by the “Talisman” Expedition. To this form the name 
of Pseudaster is assigned, but no description or diagnosis has yet been published, and all 
that we know about its characters is conveyed in the following brief statement :—“ Les 
Pseudaster ressemblent exactement & des Pentagonaster & cdtés légérement concaves ; 
leurs organes cribriformes sont rudimentaires, et leur plaque apicale grande et en forme de 
cceur” (loc. cit., p. 886). 
The Cribriform Organs.—A peculiar structure, apparently associated with special 
functions, occurs in this group. So far as I am aware it is not found, at least in 
the form presented by the Porcellanasteridz, in any other starfishes. As the structure 
is very constant, and appears to furnish a reliable character, useful for classificatory 
purposes, and also to be one of considerable morphological importance, I have pro- 
posed,” for the sake of brevity, to speak of it as the “‘ cribriform organ.” 
The structure in question is situated on the marginal plates in the interbrachial 
are; and the number of the supposed organs, which is constant in a species, may 
vary from one to more than a dozen in each are. The following brief account will 
indicate the general character of the organ throughout the series. 
In Porcellanaster the marginal plates are of uniform thickness and form a level 
plating, the successive plates fitting close together, and are not separated by any 
vertical furrow or marginal bevelling of the plate. In a species possessing only one 
of these organs in each arc (¢.g., Porcellanaster cxruleus, Pl. XX.), the structure 
about to be described is located in the median interradial line (fig. 3), and consists 
of a number of greatly compressed spinelets or lamelle arranged in vertical parallel 
lines (fig. 4). Hach of the lines thus formed is equal in length to the height 
of the two series of marginal plates, and is invested with membrane. Ten or more such 
lines or pseudo-lamelle are present on each side of the median interradial suture; and 
these do not stand quite perpendicular to the plane of the marginal plates, but are 
directed at a slight angle towards the median suture. At the upper or aboral extremity, 
where the organ terminates on the abactinal area, there is a grouping of the spinelets that 
belong to the abactinal membrane, which are also rather more robust here than elsewhere 
on the surface. At the lower extremity of the organ, the outer lamelle are rather shorter 
than the inner ones; and each being less than the next inward, a rounded outline is 
given to the lower or adoral extremity of the organ. Five or six flattened spinelets, 
1 Comptes rendus, (November 1885), t. ci. p. 886. 
2 Journ. Linn. Soc. Lond. (Zool.), 1883, vol. xvii. p. 215. 
