584 PALEOSTOMA MIRABILIS. 



plates (the vertex turned towards the actinal centre), with concave sides, to 

 which are attached the movable triangular valves, converging at the actinal 

 opening; the concave sides of the bare plates forming a petal round the 

 actinostome. into the central line of each of which the slit of the converging 

 triangular plates runs, forming thus a set of ten movable plates, soldered 

 to a set of basal plates, forming the pentagonal outline of the actinostome. 



The young specimens described by Loven would at first glance be taken 

 for young of Brissopsis ; they have the same cylindrical test, rounded at the 

 extremity; a slightly sinuous peripetalous fasciole ; short lateral ambulacra 

 with but few pores in the poriferous zone, with immense ambulacra] suckers 

 projecting through the pores of the odd ambulacrum ; the anal system 

 covered by comparatively few plates, and a pentagonal actinostome. The 

 anal system in Brissopsis soon becomes covered by a larger number of 

 plates, the actinostome assumes a crescent-shaped form, and a prominent 

 posterior lip is developed ; while the presence of a well developed sub- 

 anal and anal fasciole shows, in the earliest stages, even important typical 

 structural differences. It is interesting to note thai Platybrissus, which has 

 also an actinostome flush with the testj lias neither anal nor subanal fascioles, 

 nor a peripetalous fasciole, while Homolampas has only anal and subanal 

 fascioles. The spines of the young are curved, slender, slightly spathiform, 

 longest at the anterior extremity. Loven has noted the presence of tridac- 

 tvle toothed pedicellarioe. 



China ; East India Islands. 



