1692 THE VOYAGE OF H.M.S. CHALLENGER. 



roundish or polygonal dimples in Haeckeliana (PI. 1 14, figs. 1-6). In the Circo- 

 gonida, however, the surface is panelled, with regular or irregular polygonal plates, 

 often separated by high prominent crests (PI. 115, figs. 4-9, &c.). Numerous simple 

 thin needles of silica lie tangentially disposed and irregularly scattered in the por- 

 cellanous or cement-like substance of the thick shell-wall. Under a strong lens 

 this substance appears finely punctate, being probably pierced by numerous very fine 

 pores. The surface of the shell is therefore minutely roughened, and often appears quite 

 black under the microscope, by the adhesion of innumerable fine air-bubbles. In all 

 Circoporida, as well as in the Tuscarorida, the porcellanous shell has not the perfect 

 transparency of the purely siliceous shells of other Radiolaria ; its refractive power is 

 somewhat different from the latter ; it becomes deeply stained by carmine and browned 

 by fire, and seems therefore to consist of a peculiar carbonic silicate. 



The shell in all Circoporida exhibits a simple, excentric mouth, which corresponds 

 to the main osculum of the central capsule. The excentric position of this shell-mouth 

 has no influence on the regular form of the shell. The mouth is either circular or 

 polygonal, usually about as large as a basal circle of pores. It is always armed with 

 prominent conical or pyramidal teeth, the number of which is variable and seems to 

 depend partly upon the number of the shell-faces or the radial spines. Circoporus 

 has usually four cruciate teeth (PL 115, fig. 1 ; PI. 117, figs. 4, 5). Circorrhegma 

 exhibits a pentagonal mouth with five teeth (PI. 117, fig. 2). Circogonia possesses 

 a hexagonal mouth with six teeth (PI. 115, fig. 8; PI. 117, fig. 1). Circospathis 

 has no constant number ; some species have five teeth (PI. 115, fig. 4), others four 

 (PI. 115, fig. 10), others nine (PI. 117, fig. 3a). Circostephanus is also variable; 

 one species exhibits eight teeth (PL 116, fig. 3a), another ten, another twelve. In 

 Haeckeliana (PL 114, fig. 3) the teeth are smaller and more numerous. Usually the 

 teeth arise vertically from the surface of the shell and are spinulate ; more rarely their 

 points are directed towards the centre of the mouth. Sometimes numerous thin needles 

 arise between the teeth (PL 115, fig. 10). 



The radial spines of the Circoporida are tubular, usually cylindrical and conical 

 at the thickened base, more rarely prismatic or slenderly pyramidal, with three or more 

 edges; sometimes the edges are' spirally wound around the axis (PL 115, fig. 6). 

 In the majority of species their length is about equal to the diameter of the shell ; 

 often they are somewhat shorter, more rarely longer. Their tubular structure is always 

 the same as in the Tuscarorida ; the wall of the tube is thick, and in the axis of its cavity 

 lies an axial cord or funicle, which is connected with the wall by numerous horizontal, 

 simple, or branched threads (PL 115, figs. 6, 7). The axial funicle itself is either a 

 simple thread of silica, or a strand, composed of three to fifteen or more filaments, which 

 are closely twisted like a rope around the axis of the spine, and arise separately from 

 the bridges between the pores of the basal circle (PL 115, figs. 7, 9). The number of 



