DIAS. 



[ 247 ] 



DIATOMACE^. 



DIAS, Lillj. — A geuus of Copepoda (En- 

 tomostraca). 



D. lonffiremis. Cumberland. 



BiBL. Bradv, Copepoda {Ray Soc.) 5. 50. 



DIASTOT()RA, Lamouroux.— A genus 

 of lufundibulate Polyzoa, of the suborder 

 Cyclostomata, and family Tubuliporidfe. 



Distinguished by the iucrustiug, unde- 

 fined or foliaceous zoary ; and the alternate, 

 tubular, horizontal, immersed cells, with a 

 raised circular orifice. 



D. ohelia. Crust thin, closely adnate. 

 Three other species. 



BiBL. Johnston, Br. Zooph. 276 ; Gosse, 

 Mar. Zool. ii. 8 ; Hincks, Polyzoa, 457. 



DIA'TOMA, Dec— A genus of Diato- 

 maceae (Cohort Fragilaripe). 



Char. Frustules (in front view) linear, 

 sometimes cuneate ; at first united into flat 

 filaments, afterwards partly sepai-ating so 

 as to remain connected by the generally 

 alternate angles only, and thus forming a 

 zigzag chain. 



Filaments either free or fixed by a stipes. 

 Frustules prismatic, without rittae ; valves 

 with transverse continuous strife (costae) 

 and intermediate finer striae, not always 

 visible by direct light ; ends of the striae 

 extending into the front \'iew. 



Five British species : 



D. vulyare (PI. 16. fig. 26 : a, side view ; 

 b, front view). Fixed by an inconspicuous 

 stipes; frustides rectangadar, oblong ; valves 

 elliptical, contracted and obtuse at the ends; 

 striae evident; length of frustules 1-430". 

 Freshwater. 



D. ehngatuin. Frustules very slender, 

 slightly attenuated towards the middle ; 

 valves linear, evidently' striated, tumid 

 and roimded at the ends ; length 1-280". 

 Freshwater. 



D. grande. Valves linear, constricted near 

 the rounded ends ; costae evident. Fresh- 

 water. 



D. hycdinum. Filament of numerous 

 frustules; valves linear-elliptical, ends sub- 

 acxite, striae obscui-e. Marine. 



D. minimum. Fnistules two or three ; 

 valves elliptical, ends roimded ; striae ob- 

 scure. Marine. 



BiBL. Ralfs, Ann. N. II. 1843, xi. 449; 

 Kiitzing, Bac. ^' Sp. Alg. 16; Smith, Br. 

 Diat. ii. 38 ; Eabenhorst, Fl. Alg. i. 



piATOMA'CEvE.— A family of Confer- 

 void Algae, of very peculiar character, con- 

 sisting of microscopic brittle organisms, 

 found in almost all fresh, brackish, or salt 

 water; sometimes forming a uniform yel- 



lowish-brown layer on the bottom of the 

 water, at others adhering to various water- 

 plants, decaying stems, stones, &c., or scat- 

 tered between the filaments of Confervae 

 and marine Algas &c. They also occur 

 among Mosses, Oscillatorice, and on damp 

 ground. 



The individual cells of the Diatomaceae 

 are called frustules or testules, and are fur- 

 nished with a coat of silica (Cytioderm). 

 This consists of two usually symmetrical 

 portions or valves comparable to those of a 

 bivalve shell, which are in contact at their 

 margins with an intermediate piece (the 

 hoop), variable in breadth, according to age 

 &c. When this is very narrow, it forms a 

 mere junction line, and is called the line of 

 suture ; and that aspect of the frustules in 

 which this is turned towards the observer 

 forms the front or front view : primary side, 

 Kiitzing; secondarv side, Rabenh. (PI. 15. 

 fig. 7 ; PI. 16. figs.^ 9 a, 30 b). That aspect 

 of the frustides in which the surface of the 

 valves is turned towards the observer, forms 

 the side or side-view of the frustules : 

 secondary side ; Kiitzing, primary side, Ra- 

 benh. (PI. 15. fig. 6; PI. 16. fig. 30 a). 



_ The separate valves are of various forms : 

 circular, oblong, elliptical, linear, saddle- 

 shaped, boat-shaped (navicular), undulate, 

 sigmoid, &c. (Pis. 15, 16, 17); and their 

 surfaces exhibit various more or less delicate 

 sculpturings and markings, in the form of 

 bands, lines either parallel, radiate, or cross- 

 ing each other, and dots, or a cellular 

 (areolate) appearance. 



These markings are in general not well 

 seen, and in some cases cannot be seen at 

 all, until the valves have been properly pre- 

 pared. They are of special interest, not 

 only on account of their extremely beautiful 

 delicacy and symmetiy, but because they 

 are used as tests for the quality of the 

 object-glasses in regard to angidar aper- 

 ture. The nature of the markings is de- 

 scribed under the individual genera ; and 

 the modes of viewing them will be spoken 

 of further on. 



In some the hoop is a simple filament, so 

 curved or bent as to assume the form of 

 the section of the frustules, or the edges of 

 the valves (PL 15. fig. 11). In others, it is 

 broad, and marked like the surfaces of the 

 valves (PL 17. fig. 2 a). In others, again, 

 the hoops are numerous, flat, and arranged 

 like the leaves of a book, each with a round 

 or oval aperture in the middle, so that the 

 cavity of the frustules is divided into loculi ; 



