TH A 



THE 



green-stone; 7, amygdaloid, when in a 

 basis, generally compact, sometimes crys- 

 talline or earthy, are interspersed roundish 

 or oval bodies, composed of one or more 

 mineral substances, as in amygdaloidal 

 clay-stone or green-stone ; and 8, cavern- 

 ous, presenting numerous small cavities, 

 roundish, oval, or of various forms, in a 

 compact or granular rock. 



3. The texture of fragmentary rocks is 

 said to be, 1, brecciated, when it consists 

 of angular fragments, cemented together, 

 constituting breccia; and, 2, conglome- 

 rated, when it consists of rounded frag- 

 ments of quartz, granite, flint, or other 

 substances cemented together, forming 

 conglomerate. 



THALAMIFLO'R^ {thalamus, a bed, 

 Jlos, a flower). A sub- class of Exogenous 

 plants, having a calyx and corolla, petals 

 distinct, and stamens hypogynous. Every 

 part of the flower springs separately 

 from the thalamus, without contracting 

 cohesion with each other, as in Ranun- 

 culaceae. 



THA'LAMUS. Literally, a bed; and, 

 hence, the term is applied by botanists 

 to the dilated summit of the peduncle 

 upon which the carpels are seated ; the 

 term is synonymous with torus and with 

 receptacle. In the nomenclature of fun- 

 gaceous plants, it is the same as thallus, 

 or the bed of fibres from which many 

 fungi arise. 



THALASSI'NIANS. A family of 

 those macrurous decapods which have 

 the four anterior feet terminated by two 

 fingers, and are remarkable for the ex- 

 treme elongation of the abdomen and the 

 small degree of consistence of their in- 

 teguments. They are named from the 

 genus thalassina of Latreille. 



THALA'SSIOPHYTES {0a\dcaio':, 

 belonging to the sea, <pvr6v, a plant). 

 Sea-plants ; a general term applied to the 

 vegetable productions of the ocean, of its 

 rocks and its shores. It is synonymous 

 with the term hydrophytes, and com- 

 prises all the plants usually designated 

 as marine algae, viz. fucus, ulva, &c. 



THALLUS. A terra applied to the 

 lobed froi^d of lichens, and to the bed of 

 fibres from which many fungi arise ; it is 

 generally employed to denote the com- 

 bination of stem and leaves in the lower 

 cryptogamic plants. Thallodes is the 

 adjective used to designate any thii)g 

 arising from the thallus. 



THAU'MATROPE {daviia, a wonder, 

 TpeTTo), to turn). A philosophical toy, in 

 ■which two objects painted on opposite 

 335 



sides of a card, — as a man and a horse, a 

 bird and a cage, — are, by a quick rota- 

 tory motion, made to impress the eye in 

 combination, so as to form one picture, 

 of the man on the horse's back, the bird 

 in the cage, &c. A mental illusion 

 closely analogous to this, is produced, 

 when by a rapid and repeated transition 

 from one subject to another alternately, 

 — by a kind of intellectual thaumatrope — 

 the mind is deluded into an idea of the 

 actual combination of things which are 

 really incompatible. Whately. 



THE'CA {ei]Kr] a capsule). A term 

 applied, in Botany, to the cavity of the 

 anther, to the sporangium of ferns, to 

 the urn of mosses, &c. 



THECADA'CTYLES {dnKr], a theca or 

 hollow case, duKrvKoi, a toe). The name 

 given by Cuvier to those Geckos which 

 have the toes enlarged throughout their 

 length, and furnished below with trans- 

 verse scales, which are divided by a 

 longitudinal furrow, in which the claw 

 may be entirely concealed. 



THE'CAPHORE (ehut], a capsule, 

 (pepui, to bear). The stalk upon which 

 the ovary of plants is sometimes seated, 

 as in tacsonia. The term is synonymous 

 with gynophore, podogijniuin, &c. 



THE'CODONTS (^jjkh, a theca or 

 socket, obovi, a tooth). This and some 

 other similar terms have been employed 

 to denote the leading modifications in the 

 mode of attachment of the teeth among 

 the inferior or squaraate saurians. The 

 base of the teeth may be either ankylosed 

 to the summit of the alveolar ridge, or 

 to the bottom of an alveolar groove, and 

 supported by its lateral wall. These mo- 

 difications are indicated respectively by 

 the terms acrodont and pleurodont. By 

 a third mode of fixation, the teeth are 

 implanted in sockets, either loosely or 

 confluent with the bony walls of the 

 cavity : these. Professor Owen calls the- 

 codont lacertians. 



THECO-SO'MATA (6///k»), a theca or 

 cavity, aS^ia, a body). The name given 

 by De Blainville to the first family of 

 Aporobranchiata, or those molluscs which 

 are provided with natatory appendages, 

 without any foot properly so called, and 

 with the organs of respiration but little 

 evident. According to Mr. J. E. Gray, 

 the thecosomata constitute the first order 

 of the class Pteropoda. 



THE'COSTOMES (^/jkh, a cavity, o-t6- 

 /ia, the mouth). The name given by 

 Latreille to those insects which have a 

 suctorious mouth enveloped in a sheath. 



