T R I 



[263] 



T R I 



are very rare ; it is said some large spe- 

 cimens have been obtained from the neigh- 

 bourhood of Verona. 

 TRIDA'CTYLOUS. Having three toes. 

 TRI'GONAL. (trigonalis, Lat. ) Trian- 

 gular ; three-cornered ; having three 

 sides. 



TRIGONE'LLITES. A genus of shells de- 

 scribed, and thus named, by Mr. Parkin- 

 son. A slightly rounded, trigonal, thick 

 shell, gaping on each side. The anterior 

 margin nearly on a straight line ; the 

 posterior in a gently waving, and the 

 upper side in nearly a circular direction. 

 The outer surface of each valve thickly 

 pierced by foramina, which, passing nearly 

 through its substance, give it the can- 

 cellous appearance of bone : the inner 

 surface smooth, but marked with striae, 

 concentric with the upper margin. The 

 hinge completely linear, without teeth ; 

 there being only an appropriate surface 

 on the anterior margin of each valve, for 

 the attachment of the cartilage exter- 

 nally. Mr. Parkinson also observes, 

 " this shell, which I have presumed thus 

 to name and to dispose of, is exceedingly 

 anomalous in every respect : it is of an 

 extraordinary degree of thickness, being, 

 in some specimens, full three-eighths 

 of an inch thick, and even in young 

 specimens, full one quarter of an inch 

 thick." 



In the supplementary notes attached 

 to Prof. Buckland's Bridgewater Trea- 

 tise it is stated, " it is shown in a notice 

 read by M. Voltz to the Natural History 

 Society at Strasbourg, that the problema- 

 tical fossils known by the name of Apty- 

 chus, Trigonellites, &c , which are some- 

 times found lodged in pairs within the 

 first chamber of the ammonites, were 

 opercula connected with the foot, or 

 organ by which the animals inhabiting 

 these shells moved along the bottom of 

 the sea." 



TRIGO'NIA.^ (from rpiywvor, triangular, 

 TRI'GON. )> Gr.) A genus of marine bi- 

 TRY'GON. J valves, found both fossil and 

 living, belonging to the family Ostracea. 

 The trigonia is a triangular or suborbicu- 

 lar, inequilateral, equivalve, transverse 

 bivalve. One valve has two oblong, 

 flat, diverging, hinge teeth, transversely 

 grooved on each side ; the other has four 

 flat, oblong, diverging, hinge teeth, trans- 

 versely grooved on one side only, dis- 

 posed in pairs, receiving between their 

 grooved sides the two hinge teeth of the 

 opposite valve. Recent trigonise have 

 hitherto been discovered near Australia 

 only, in sandy mud. They are nearly 

 related to the cardium, or cockle, the 

 foot of the animal being bent, like that 



of tVif> v^l.-'~ or> 4r>wf-A apo-lp. SO as. 



upon pressure, to form a very elastic 

 organ. The species found recent has 

 been named Trigonia Margaritacea, or 

 pearl trigon, from the pearly lustre of the 

 inside of its shells. 



Fossil trigoniae differ considerably from 

 the "recent ; nine species have been de- 

 scribed by Mr. Parkinson. Three spe- 

 cies are mentioned by Dr. Mantell as 

 occurring in the Shanklin sand. Tri- 

 gons are also found in the lias, oolite, 

 and tertiary formations. 



TRILO'BATE. (from tres, three, and lobus, 

 a lobe, Lat.) Divided into three lobes 

 having three lobes. 



TRI'LOBITE. (So named from its being 

 divided into three lobes or principal 

 parts.) A family of fossil crustaceans, 

 which appear to have become extinct at 

 the close of the period during which the 

 carboniferous series were formed, no 

 traces of their remains having been dis- 

 covered in strata of a more recent pe- 

 riod. Twelve genera and fifty-two spe- 

 cies have been described. For a long 

 time, fossil trilobites were confounded 

 with insects, under the name of Entomo- 

 lithus paradoxus : several names have 

 also been given to them, derived chiefly 

 from the three lobular divisions by which 

 they are so characteristically marked, as 

 well as from their being found sometimes 

 in a coiled, sometimes in an extended 

 state. By Bromel, the trilobite was 

 named Lapis insectiferus ; by Wolster- 

 dorf, Conchitus trilobus ; by Hermann, 

 Pectunculites trilobus imbricatus ; by Da 

 Costa, Pediculus marinus ; by Linnaeus, 

 Entomolithes paradoxus ; by Baumur, 

 Trigonella striata ; and by Wilke, Ento- 

 molithus cancriformis marini. The tri- 

 lobite is often called the Dudley fossil, 

 from its having been first noticed in the 

 transition limestone near that town. Mr. 

 Bakewell states that the largest species 

 are found in the slate quarries at Angers, 

 in France, and that he possesses a speci- 

 men from that place which measures 

 seven inches in length. The following 

 details are extracted from Prof. Buck- 

 land's very interesting and copious de- 

 scription of trilobites : " The great ex- 

 tent to which trilobites are distributed 

 over the surface of the globe, and their 

 numerical abundance in the places where 

 they have been discovered, are remarkable 

 features in their history. They have 

 been found throughout all northern Eu- 

 rope, and in numerous localities in North 

 America, in the Andes, and at the Cape 

 of Good Hope. The anterior segment of 

 the trilobites is composed of a large 

 semi-circular, or crescent- shaped shield, 

 succeeded by a body composed of nu- 

 merous segments folding over each other, 



