Navicalacea.'] infusorial animalcoles. 319 



is not strictly bivalve, although capable of being separated into two 

 •corresponding portions ; but is more properly multivalve, as con- 

 sisting of two discoid portions, and of two annular valves, exactly 

 similar respectively to one another. Each discoid valve is further 

 separable into an outer, very thin, somewhat flexible, and elastic 

 membrane, on which are the characteristic spider's wcb-like mark- 

 ings. (P. 24, f. 18) ; and a silioeous frame- work, resembling 

 a circiilar gothic window. (P. 24, f. 19.) . . . It is indisputably 

 evident, that a central opening is present, partially covered inter- 

 nally by a delicate cup-like process (membrane), so as to form a 

 species of valve. The two annular valves are situated between the 

 two discoid ones ; and thus the whole shell, in its natural state, 

 resembles a circular snuff box. These valves consist of a siliceous 

 ring, within Avhich, (extending a slight distance towards the centre) 

 is an annular membrane, and when in situ, the valves are placed so 

 that the membranes of each valve are in contact ; and thus the 

 space included between the two discoid valves is partially divided 

 into two (not three) chambers." (Trans. Mic. Soc. V. 3, p. 49.) 

 Mr. Deane has supposed the presence of a short siliceous pedicle ; 

 but Mr. Shadbolt is satisfied there is none, but that all the shells 

 are sessile. The latter thinks it probable, they may propagate by 

 self-division ; yet they occur of all imaginable diameters, from 

 4-lOOOth to 14-lOOOth of an inch, and the number of rays, in each 

 individual, is by no means constant. 



It should be added, that this same observer is disposed to consider 

 them animals, and that he supposes food to be taken in through the 

 openings about the centre, not improbably by tentacula, arranged 

 like those of Madrepores. (P. 24, figs. 18 to 21.) 



AEACHifoiDiscus Joponicus. — Specific characters unknoAvn. So 

 named, because found on Algse, from Japan ; used as an article ol food. 



Mr. Shadbolt intimates the existence of other species, found in 

 Ichaboe guano ; Algse from the Cape of Good Hope; from I^orth 

 America, &c. 



Genus Asterodiscus (S. Johnson.) — Lorica simple, bivalve, siliceous, 

 circular ; fi^om the side bi- convex, not forming chains. Umbilicus 

 smooth, nearly circular ; divided from the margin by imperfect septa, 

 of Avliich every two unite half way to centre ; the rest radiating 



