H L 



[ 217 ] 



H K 



variety of calcareous spar. The 

 Matastatique of Haiiy. 

 HOLOPTY'CHIUS. A genus of fossil 

 fishes, discovered in the old 

 red sandstone formation. The 

 name has been assigned to ichthyo- 

 lites of this genus by M. Agassiz 

 from the folds of the scales. A 

 species has been named H. Nobilis- 

 simus, in honour of the Eev. James 

 Noble, who discovered a splendid 

 specimen, measuring two feet four 

 inches by twelve inches, in the old 

 red sandstone at Clashbinnie, in 

 Scotland. 



"The generic characters," says 

 M. Agassiz, "consist in the peculiar 

 structure of the scales, the enamel- 

 led surface of which is marked by 

 large undulating furrows. Another 

 characteristic feature is in the dis- 

 tant position of the ventral fins, far 

 removed towards the tail, and much 

 nearer the arms and anal fins than 

 in any other genus of the family of 

 ganoids." Murchison's Silurian 



The Holoptychius is the most 

 characteristic and most abundant 

 ichthyolite of the upper old red 

 sandstone. Agassiz states that the 

 Holoptychius of the coal measures 

 is the largest of all osseous fishes. 

 Hugh Miller. 



HOLOTHT/BIA. A zoophyte belonging 

 to the order Pedicellata, class 

 Echinodermata. The holothuria is 

 covered with a thick coriaceous 

 skin, which, by means of longitu- 

 dinal and circular bands of muscu- 

 lar fibres, the animal can shorten 

 or lengthen at pleasure. The body 

 is oblong, and open at each end; 

 numerous tentacula surround the 

 mouth. There are many species. 



HO'LMITE. A mineral, a variety of 

 carbonate of lime, named after Mr. 

 Holme, who analysed it. Its con- 

 stituents are lime, carbonic acid, 

 oxide of iron, silica, alumina, and 

 water. 



HOMACA'NTHUS. A genus of ichthyo- 



lites of the old red sandstone, one 

 species of which, H. Arcuatus, has 

 been described by Agassiz. 



HO'MALONOTUS. The name assigned 

 by Mr. Konig to a genus of trilo- 

 bites, in which the longitudinal 

 division into lobes is scarcely per- 

 ceptible. Four species have been 

 described, namely, H. Knightii, 

 H. Ludensis, H. Delphinocephalus, 

 and H. Herschelii. These are all 

 found in the Upper Silurian rocks. 



HO'MOCERCAL. See Heterocercd. 



HOMOGENEITY. (homogtneite, Fr. 

 omogeneitd, It.) Of the same 

 nature; having the same nature 

 throughout. 



HOMOGE'NEAL. } (ojioyevrjs, Gr. homo- 



HOMOGE'NEOFS. j gene, Fr. omogeneo, 

 It. ) Similitude of kind ; sameness 

 of nature. 



HOMO'LOGOUS. (homologue, Fr. omologo y 

 It. 6,110X0709, Gr.) Having the 

 same manner or proportions. 



HOMOTHO'RAX. A genus of fossil 

 fishes of the old red sandstone. 



HONE, (hcen, Sax.) Whetstone slate. 

 A variety of talcy slate, containing 

 particles of quartz : when these 

 particles are exceedingly minute, 

 and the slate possesses a certain, 

 degree of hardness with a uniform 

 consistence, it yields hones of the 

 best quality. Kirwan gave to this 

 mineral the name novaculite, from 

 novacula, the Latin for a razor. 



HO'NEY- STONE. Pyramidal mellite. 

 The Honigstein of Werner ; Mellite 

 of Haiiy and Erongniart ; and 

 Mellilite of Kirwan. It is pyra- 

 midal, its primitive form being a 

 pyramid of 118 4' and 93 22'. 

 It was first discovered in Thuringia, 

 between the layers of wood-coal. 

 It is of a honey-yellow colour, 

 sometimes a little tinged with 

 brown. Transparency considerable. 

 When heated it whitens, and in 

 the open air burns without being 

 sensibly charred. It yields neither 

 flame, smoke, nor odour. It is 



