454 



MOLLUSCA 



PHYLUM V 



Beaks prosogyrous ; hinge plate very broad and massive, without laterals ; the two 

 cardinal teeth separated by a deep socket ; anterior adductor scar small, semilunar, 

 in front of the anterior cardinal ; posterior scar longer, less distinct, situated on an 

 elevated or thickened radial ridge. The oldest Devonian species {M. cucuUatus Goldf.) 

 has amorphous cardinals and a smooth rounded shell (Euviegalodon). The Triassic 

 species sometimes attain a large size, have a radial posterior ridge, smooth teeth, and 

 divided right posterior cardinal teeth {Neomegalodon Gilmb.). They are extraordinarily 

 aljundant in the Dachstein limestone and Hauptdolomite of the Northern Alps, and 

 are also plentiful in the Raibl and Rhaetic beds of the Southern Alps.^ 



Pachyrisma Morr. and Lye. {Pachyme.galodon Giimb.). Like Megalodon, but with 

 a larger anterior adductor scar, a rounded anterior tooth before the cardinals, and a 

 strong posterior lateral. Trias to Upper Jura. 



Durga Bohm. Like Pachyrisma, but without an elevated area at the posterior 

 adductor. Lias. 



Protodiceras Bohm. Lias. Dicerocardium Stoppani. Rhaetic. 



Megalomus Hall. Silurian ; North America. 



Family 28. Unionidae Fleming. 



Shell equivalve, dimyarian, typically schizodont, ivith pseudocardinals and laterals 

 if dentiferous ; consjncuously nacreous ; beaks usually sculptured ; ligament opisthodetic, 

 external; lobes of the mantle generally united to form an anal sipjhon, but the functional 

 branchial siphon ahoays incomplete below; foot compressed, keeled, large, rarely with a 

 feeble byssus ; usually dioecious; the young having a specialised prodissoconch 

 (glochidium) and a distinct nepionic stage. Fluviatile. Trias to Recent. 



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

 the Cretaceous and Tertiary. The origin of the family has been sought in the Trigoniidae,^ 

 which have a very similar ontogeny as a group ; in Trir/onodus ^ and related forms ; and by 

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

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

 though there is mucli probability that each of tliese groups bears a certain amount of relation- 

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



Recent studies by Simpson and Ortmann of the anatomy of living genera of Unionidae 

 have shown that considerable differences exist as regards the soft parts, more especially the 



Fio. 699. 

 Unio stachei Neumayr. Pliocene (Congerian stage) ; Sibinj, Slavonia. ;;>, Adductor ; x. Pedal scar. 



marsupial apparatus (that part of the gills which contains the glochidia) and the shape of 

 the glochidia. These differences permit of an improved system of classification for the 

 modern forms, but as the shape of the shell is not at all correlated with the natural divisions 

 indicated by the structure of the soft parts, it is impossible to apply this system with 

 certainty to fossil Unionidae. 



Unio Retzius (Fig. 699). This, the typical genus, originally included also the pearl 



1 Tausch, L. vq)i,_ tJber Coiiohodus, etc. Abhandl. geol. Reichsanstalt, 1892, vol. xvi. 



2 Neumayr, M., Uber die Herkunft der Unioniden. Sitzber. Akad. Wiss. Wien, 1889, xcviii. 



3 Wiihrmann, S. von, Uber die systematische Stclluiig der Trigoniden uud die Abstamniung 

 der Nayadeu. Jahrb. geol. Reichsanst., 1893, vol. xliii. 



