KEPOKT 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 ...... CTENODisciNiE. 



a. A single genus ........... . Ctenodiscus. 



Another genus is ranked amongst the Porcellanasteridse by M. Perrier 1 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 a des Pentagonaster a cote's legerement concaves ; 

 leurs organes cribriformes sont rudimentaires, et leur plaque apicale grande et en forme de 

 cceur" (doc. 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 Porcellanasteridse, 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, 2 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 

 arc ; and the number of the supposed organs, which is constant in a species, may 

 vary from one to more than a dozen in each arc. 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 {e.g., Porcellanaster casruleus, PI. 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 lamellae arranged in vertical parallel 



Hues (fig. 4). Each 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-lamellae 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 orgau, the outer lamellae 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. 



' Journ. Linn. Soc. Lond. (Zool.), 1883, vol. xvii. p. 215. 



