246 TRIGONIIDA. 
are supposed to be migratory, as dredging for them is very 
uncertain, though they abound in some parts of Sydney Harbor. 
Trigoniz appeared in Devonian and Liassic strata, became 
numerous in the Jurassic and Cretaceous, and have since con- 
tinued to decline. Australia, where so many others of earth’s 
oldest forms continue to exist, furnishes the only living species. 
Trigoniz are divided according to form and sculpture into 
several sections, all of which are extinct except the last. 
Scaphoidea. T. navis, Lam. (cxxv,19). lias; Alsace. 
Undulate. 7. undulata, Fromh. Jura. 
Costate. T. costata, Lam. (cxxv, 20). Oolite; Switzerland. 
Glabre. JT. longa, Agassiz (exxv,21). Neocomian; Neufchatel. 
Quadrate. T. Parkinsonii, Agassiz (cxxv, 22). Portlandian ; 
Besangon. 
Scabre. TT. scabra, Lam. (cxxv, 23). Cretaceous. 
Byssiferse. TJ. carinata, Agass. Neocomian. 
Clavellate. (Myophorella, Bayle.) Z.clavellata,Sowb. Jura. 
Pectinate. 7. pectinata, Lam. Australia (living). 
Scuizopus, King, 1848. 
Syn.—Axinopsis, Tate. Megalodus (part), Goldf. 
Distr.—Fossil,20 sp. Upper Silurian—Muschelkalk; United 
States, Europe. 8S. Schlotheimi, Geinitz (exxvii, 56). 
Shell trigonal, rounded in front, attenuated behind; rather 
thin, smooth, with an obscure, oblique ridge; ligament external ; 
hinge-teeth 2°3, smooth, rather small; anterior adductor slightly 
impressed, removed from the hinge, with a pedal scar close to it; 
pallial line simple. 
NEOSCHIZODUS, Giebel, 1856. Shell similar to Schizodus; hinge 
of the left valve with a large posterior, subterminal, cardinal 
tooth, posteriorly prolonged, parallel to the fulecra, which are 
distinct from the margin itself, and with an anterior terminal 
eardinal; right valve with a marginal elongated posterior and a 
subterminal shorter, but thicker, anterior cardinal tooth. S. levi- 
gatus, Giebel. Triassic; Germany. 
MYOPHORIA, Bronn, 1830. (Cryptina, Boue.) Shell trigonal, 
umbones turned forwards; obliquely keeled ; smooth or sculp- 
tured ; teeth 2°3, striated obscurely, centre tooth of left valve 
simple, anterior of right valve prominent; mould like Trigonia. 
M. decussata, Munst. (cxxvii, 57), has a lateral tooth at the dorsal 
angle of the left valve. Fossil, 16 sp. Trias; Germany, Tyrol. 
PRISCONATA, Conrad, 1867. Shell ovate ; hinge in the left valve 
with two cardinal teeth, the anterior compressed, angular, 
oblique, with an anterior pit; the posterior broad, smooth, con- 
vex, triangular, situated under the beak and directed posteriorly. 
emarginated at its end; no lateral teeth; muscular impression 
