CHALAZA. 



[ 131 ] 



CHALK. 



posterioi'ly a rills' of dark prickles ; leng'th 

 1-620". 



C. aspera (PI. 24. fig. 26 c). Oblong, 

 rounded at each end, hispid with short setae; 

 posterior prickles scattered without order; 

 length 1-550". 



C. ? pyrifee. Oblong cylindrical, ends 

 rounded; seta? slender and elongate; no 

 posterior prickles; length 1-1100", breadth 

 half the length. Fossil in flint. Ehrenberg 

 questions whether this is not a Xanthidium. 



BiBL. Ehr. Infus. p. 250; Duj. Infus. 

 p. 328. 



CHALAZA (in plants). — The term ap- 

 plied to the base of the nucleus of ovules, 

 where the substance of the former is con- 

 tinuous with the coats, and where the vas- 

 cular cord derived from the placenta termi- 

 nates (fig. 120, ch). 



Fig. 120. 



Ch^ 



Section of an anatropous ovule. /, funiculus ; r, raphe ; 

 ck, chalaza; p, external coat or testa; s, internal coat or 

 tegmen ; n, nucleus. 



CHALTMUS, Biirm.— A genus of Crus- 

 tacea, of the order Sijihonostoma, and family 

 Caligidse. 



Char. Fourth paii' of feet slender, of only 

 one branch, and serving for walking ; frontal 

 plate with a long and slender prehensile ap- 

 pendage arising from the middle of its ante- 

 rior surface. 



C scombri. Found upon the mackerel, 

 and upon species of Caligus, of which it has 

 been supposed to be the young; length 

 about 1-6". 



BiBL. Biirmeister, N.A.Acad. N. C.Bonn, 

 xvii, ; Baird, British Entomostraca, p. 278. 



CHALK. — The earthy form of carbonate 

 of lime. The application of the microscope 

 to the examination of chalk brought to light 

 the interesting fact, that this substance has 

 not had its origin in chemical precipitation, 

 since it contains abundance of the inorganic 

 remains of marine animals and plants, princi- 

 pally the former. 



Many of these remains are not micro- 

 scopic, as the remains of birds and reptiles, 

 the shells of Mollusca, Echinodermata, the 

 polypidoms of Zoophytes, &c., hence their 

 consideration does not come within our pro- 

 vince ; yet it must be remembered that their 



microscopic structure is to a certain extent 

 characteristic, so that the class, order, or 

 even the more minute division of the animal 

 kingdom to which they belong may be dis- 

 covered. See Bone and Shell. 



The chief microscopic constituents of the 

 calcareous formations examined by M. Ehren- 

 berg, viz. chalk, chalk-marl, compact lime- 

 stone, and nummulitic limestone, were 

 found to be shells of Foraminifera, spicules 

 of Sponges, the valves of the Diatomaceae, 

 and peculiar bodies called crystalloids. 



The Foraminifera found by M. Ehrenberg 

 in the Brighton chalk were — Rotalia glo- 

 bulosa* and turyida'^, Textidaria aciculata, 

 aspera*, globulosa and striata, and Turbimi- 

 Uiia italica (?). The * denotes those most 

 common. 



Those in the Gravesend chalk were — 

 Rotalia turgida*, Rosalina globularis, Ro- 

 talia globidosa*, Textularia aspera^, globu- 

 losa"^, and striata. 



M. D'Orbigny enumerates 22 species of 

 Foraminifera from the English chalk. The 

 Diatomaceae found by Ehrenberg in the cal- 

 careous formations belonged to the genera, — 

 Actinocyclus, Actinoptychus, Amphitetras, 

 Biddidphia, Cocconema, Coscinodiscus, Eu- 

 notia, Ejnthemia, Eupodiscus, Fragilaria, 

 Melosira, Grammatophora, Navicula, Stria- 

 fella, Synedra, Tessella, and Triceratium, 

 with four species of Dictyocha. 



The cementing material of chalk consists 

 of very minute, numerous and remarkable 

 bodies, called crystalloids (PI. 19. fig. 15). 

 Thev are elliptical, or rounded and flattened, 

 from 1-10,000 to 1-2500" in length, the 

 most numerous perhaps 1-3000" ; some of 

 them consist of a simple ring (a) ; in 

 others this is marked with pretty regular 

 transverse lines, so as to make it appear 

 jointed {b) ; in others, again, there is a 

 thinner central portion, often exhibiting one 

 or more granules (c). M. Ehrenberg re- 

 gards these as arising from the disintegra- 

 tion of the microscopic organisms forming 

 the chalk into much more minute calcareous 

 particles, and their reunion into regular ellip- 

 tical plates (or disks) by a peculiar process, 

 differing essentially from, and coarser than 

 that of crystallization, but comjmrable with 

 it ; one probably preceding all slow crystal  

 line formation, and causing, but not alone, 

 the granular state of solid inorganic matter. 

 The best method of examining chalk for 

 Foraminifera is this : place a diop of water 

 upon a glass slide, and put into it as much 

 finely-scraped chalk as will cover the point of 



k2 



