246 TRIGONIID^. 



are supposed to be migratory, as dredging for them is verj?^ 

 uncertain, though they abound in some parts of Sydney Harbor. 



Trigonise appeared in Devonian and Liassic strata, became 

 niimerous 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. 



Trigonijfi 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. 



Undulatse. T. undulata, Fromh. Jura. 



Costatse. T. costata, Lam. (cxxv, 20). Oolite ; Switzerland. 



Glabree. T. Zongra, Agassiz (cxxv,21). Neocomian; Neufchatel. 



Quadratse. T. Parkinsonii^ Agassiz (cxxv, 22). Portlandian ; 

 Besan§on. 



Scabrffi. T. scabra, Lara, (cxxv, 23). Cretaceous. 



Byssiferae. T. carinata, Agass. Neocomian. 



Clavellatse. ( Myophorella, Bayle.) T. clavellata^Sowh. Jura. 



Pectinatse. T. pectinata, Lam. Australia (living). 



ScHizoDUS, King, 1848. 



Syn. — Axinopsis, Tate. Megalodus (part), Goldf 



Bistr. — Fossil, 20 sp. Upper Silurian- — Muschelkalk ; United 

 States, Europe. S. Schlotheimi, Geinitz (cxxvii, 56). 



Shell trigonal, rounded in fiont, 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, Gicbcl, 1856. Shell similar to Schizodus; hinge 

 of the left valve with a large posterior, subterminal, cardinal 

 tooth, posteriorly prolonged, parallel to the fulcra, which are 

 distinct from the margin itself, and with an anterior terminal 

 cardinal ; right valve with a marginal elongated posterior and a 

 subterminal shorter, but thicker, anterior cardinal tooth. S. Isevi- 

 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. 



PRiscoNAiA, Conrad, 186*7. 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 



