TEST-OBJECTS. 



[ 762 ] 



TETRANYCIIUS. 



Didymohelix (PL 1. fig. 10 a, b, c, d). 

 The filaments should be mounted in solution 

 of chloride of calcium, or in Canada balsam. 

 It is very difficult to display the component 

 fibres of this beautiful object when in bal- 

 sam. It also forms a good test of magni- 

 fying power. 



Didymoprium (PI. 1. fig. 11). The lon- 

 gitudinal lines upon the cells require con- 

 siderable magnifying power. 



The pygidium of PULEX. An excellent 

 test-object, mounted in as small a quantity 

 of balsam as possible. Dujardin represents 

 the rays upon the disks as round, like so 

 many beads, whereas they are wedge-shaped 

 with the bases outwards. 



The valves of the JDiatomacece. It is a 

 difficult matter to show the lines upon 

 Grammatophora marinawiih an object-glass 

 of 110 of angular aperture, requiring ex- 

 tremely oblique light ; but many of those 

 mentioned above are far more difficult. 



The ultimate jibrillce of muscular fibre 

 (PI. 22. fig. 36). Mounted in liquid. 

 Kolliker represented them as beaded 

 (fig. 36 c) ; they have also been repre- 

 sented a.s in a : probably both these inac- 

 curacies arise from imperfect adjustment, 

 and from their immersion in too mnch 

 liquid. Their true structure is figured in 

 b, d, e. 



Noberfs test-lines. See NOBEBT'S LINES. 



We have omitted to notice several test- 

 objects, as the scales of some insects, a 

 minute globule of mercury, &c., because the 

 former have been so obscurely described 

 that we are unable to comprehend in what 

 the test-structure consists ; and the test- 

 appearances presented by the latter viewed 

 as an opaque object are inappreciable to one 

 unaccustomed to the use of the microscope, 

 by whom mainly are remarks upon test- 

 objects required. See on this matter, 

 Wenham, in Beale's How &c. p. 436. 



Chevalier's test-object consists of the 

 scales of Pontia brassicce (PI. 34. fig. 24), 

 the granules being rendered distinct ; this 

 is a test for definition. 



Mohl recommended the scales of Hip- 

 parchia janira for testing penetrating 

 power ; pollen-grains, the scaly elytra of the 

 diamond-beetle, or bat's hair, for definition. 



Schacht's test-object consists of the scales 

 of Hipparchia janira (PI. 1. fig. 9c), a test 

 for moderate angular aperture and oblique 

 light. 



BIBL. That of the INTRODUCTION, and 

 of ANGULAR APERTURE. 



TE'THEA, Lam. A genus of marine 

 Sponges. 



Char. Solid and compact,rounded, covered 

 with a skin ; without sensible pores ; inte- 

 rior fleshy, with acicular and globulo-subu- 

 late spines (PI. 45. fig. e). 



BIBL. Johnston, Br. Spong. 81; Gosse, 

 Mar. Zool. i. ; Huxley, Ann. N. H. 1851, 

 vii. 370. 



TETMEM'ORUS, Ralfs. A genus of 

 Desmidiacese. 



Char. Cells single, simple, elongated, 

 straight, cylindrical or fusiform, constricted 

 in the middle ; segments emarginate at the 

 ends. 



Sporangia square or round. 



T. granulatus (PI. 14. figs. 33, 34) . Cells 

 fusiform both in front and side view, ends 

 colourless and lip-like j dots irregular. 

 Length 1-130". 



T. Icevis (PL 14. fig. 35, in conjugation). 

 Cells in front view somewhat tapering, ends 

 truncate ; side view fusiform ; dots none, or 

 very indistinct (under ord. ilium. ). Length 

 1-860". 



T. Brebissonii. Dots in longitudinal 

 rows. 



BIBL. Ralfs, Br. Desmid. 145 ; Rabenht. 

 Alg. iii. 139. 



TETR ACHASTRUM = Micrasterias pt. 



TETRACY'CLUS, Ralfs. A genus of 

 Diatomaceae. 



Char. Frustules compound, aggregated 

 into a filament, in front view broadly tabular 

 with longitudinal interrupted vittae ; valves 

 inflated on each side in the middle. 



Valves with coarse transverse striae. 



T. Tkienemanni, Ehr. (lacustris, Ralfs) 

 (PL 17. fig. 28). Valves rounded or sub- 

 acute at ends, inflations rounded. 



T. emarginatus. As the last, but valves 

 constricted towards the rounded and sub- 

 apiculate ends, and the inflations eniar- 

 ginate. 



BIBL. Ralfs, Ann. N. H. 1843, xii. 105 

 Kutzing, Sp. Alg. 118; Smith, Br. Diat. ii. 

 37 ; Rabenht. Alg. i. 302. 



TETRAGON'iCEPS,Brady. A genus of 

 Copepodous Entomostraca. T. malleolata, 

 in dredgings. (Brady, Copep. ii. 65.) 



TETRAMTTUS,Perty. A genus of Fla- 

 gellate Infusoria. Free, changeable, tapering 

 behind, truncate in front; flagella four. 

 Three species ; in stale water and infusions. 

 (Perty, Lebensf. 170 ; Kent, Inf. 313.) 



TETRAN'YCHUS, Duf. A genus of 

 Tronibidina (Acarina). Spinning-mites. 



