TESSELLATED. (Wrought in chequer-work). A term applied to the colouring of shells, when arranged in regular defined patches like a tessellated pavement. TESTACELLA. (_Testa_, a shell.) _Fam._ Limacinea, Lam. and Bl.--_Descr._ Haliotoid, compressed; aperture wide, oblique; columella flat, oblique; spire short, flat, consisting of less than two whorls.--_Obs._ This shell which is extremely small compared with the animal, is placed upon its back, near the posterior extremity. The animal is found in some of our gardens, and very much resembles the common garden slug. Fig. 261, T. Haliotoidea. TESTACEOUS. (_Testa_, a shell.) Shelly. Testaceous Mollusca, are soft animals having shells. A testaceous operculum is one composed of shelly matter. TETRACERA. Bl. The first family of the order Polybranchiata, Bl. containing no genera of testaceous mollusca. TEXTILIA. Sw. A sub-genus of Conus, consisting of Conus bullatus, &c. Sw. Malac. p. 312. TEXTULARIA. Defr. A genus of microscopic Foraminifera. THALAMUS. Montf. A genus described as resembling Conilites, but curved and granulated. THALLEPUS. Sw. A genus of "Aplysianiæ," Sw. thus described: "Body more slender and fusiform;" (than Aplysia,) "the lobes of the mantle short, and incapable of being used for swimming; tentacula two, large, ear shaped; eyes not visible. T. ornatus, _Sw._ Sp. Nov." Sw. p. 359. THALLICERA. Sw. A generic name under which Swainson distinguishes AMPULLARIA Avellana, Auct. THECIDIUM. (_Thecas_, a box.) _Fam._ Brachiopoda, Lam. _Order_, Palliobranchiata, Bl.--_Descr._ Lower valve concave, sub-trigonal, with the umbo produced into a triangular, slightly incurved beak, and with two short, pointed processes advancing from beneath the umbones; upper valve flat, rounded square, with a short, blunt appendage, formed to fit between the tooth-like process of the other valve; its inner surface ornamented with symmetrically curved ridges. THECOSOMATA. Bl. The first family of the order Aporobranchiata, Bl. containing the genera Hyalæa, Cleodora, Cymbulia, Pyrgo. THELICONUS. Sw. A sub-genus of Conus. Lardn. Cyclop. Malac. p. 312. THELIDOMUS. Sw. A generic name under which Swainson has described a division of the genus Helix, and which he has also used to designate a genus in the family of "Rotellinæ," founded upon an aggregate of loose particles collected and agglutinated in a spiral form by the larva of an insect. Sw. Malac. p. 330 and 353. THEMEON. Montf. A genus of microscopic Foraminifera. THEODOXUS. Montf. A division of the genus Nerita. Fig. 324, N. virginea.