ORDER I 



I'lMONoDKSMACKA 877 



in front of the anterior cardinal ; posterior scar longer, le.-s di-tin<-t, .-ituated on an 

 elevated oi- thirkened radial ridge. The oldest Devonian >]. je- M. ,-</. /////////. (loldf. 

 has amorphous cardinals and a smooth rounded shell (Sumegolodon). The Tria-.-i<- 

 species sometimes attain a large >i/e, have a radial posterior ridge, smootli tct-th, ami 

 divided right posterior cardinal teeth (Neomtgalodon, dumb.; They are extraordinarily 

 abundant in the Dachstein, Limestone, and Haujitdolomite of tin- Northern Alps, and 

 are also plentiful in the Raibl and llhaetic beds of tin- Southern Alj.' 



Pachyrisma, Morr. and Lye. (Pachymegalodon, Giimb.) Like Mi<in]i>ilni), but with a 

 larger anterior adductor scar, a rounded anterior tooth l.rtore the cardinals, and a 

 strong posterior lateral. Trias to Upper Jura. 



Durga, Bohm. Like Pachyrisma, but without an elevated area at the po-terior 

 adductor. Lias. 



Protodicerox, Bohm. Lias. Dicerocardium, Stoppani. Rhaetic. 



Family 28. Unionidae. Fleming. 



Shell equivalve, dimyarian, typically Schizodont, with pseudocardinals and laterals 

 ifili/itiferous; conspicuously nacreous; beaks usually sculptured ; ligament opisthodetic, 

 external; lobes of the mantle united to form an anal siphon, but the functional br(tn<-}iinl 

 siphon usually incomplete below ; foot compressed, keeled, large, rarely with a feeble byssus ; 

 usually dioecious ; the young having a distinct nepionic stage (glochidium). Flud-iHI-. 

 Trias to Recent. 



Typical Uniones make their appearance in the Trias of Texas, but are not abundant until 

 the Cretaceous arid Tertiary. The origin of the family has been sought in the Trigoniidae, z 

 which have a very similar ontogeny as a group ; in Trigonodus* and related forms ; and by 

 Pohlig in the Triassic Uniona. An older view recognises the Carboniferous Anthracosia and 

 other Cardiniidae as probable ancestors. The weight of evidence is in favour of the latter, 

 though there is much probability that each of these groups bears a certain amount of relation- 

 ship to the present family, which will be better realised when more evidence is obtainable. 



Unio, Retzius (Fig. 650). This, the typical genus, was founded on the pearl 

 mussel (Mya margaritifera, Linn.), in which the posterior laminae of the hinge are 



Fir;. 

 Unio Stachci, Xeuinayr. Pliocene (Congerian stage); Sibinj, Slavonia. 7), Adductor; JT, Pedal MMI-. 



obsolete. The majority of species, however, have amorphous, heavy, radial, pseudo- 

 cardinal and lateral teeth on the hinge; the shell is variable in form and ornamenta- 

 tion, some species having strongly marked sexual differences in the sludl. Most of 

 the species are pearly, with a conspicuous brown or greenish periostracum ; the 

 anterior adductor scars are high, and the pedal scars conspicuous. 



1 Tausch, L. von, Ueber Conchodus, etc. (Abhandl. geol. Reichsanstalt, XVII.), 1892. 



2 Neumayr, J/., Ueber die Herkunft <ler Unioniden (Sitzber. Wien. Akad. XCVIII.), 1889. 



3 W&hrmann, S. von, Ueber die systematische Stellung der Trigonidem und die Abstannuunu' 'l'-i 

 Nayaden (Jahrb. geol. Reichsanst. Bd. XLIII.), 1893. 



