454 



MOLLUSCA 



PflYLUM V 



Beaks prosogyrous ; hinge plate veiy broad and massive, without laterals ; tlie two 

 cardinal teetli separated by a deep socket ; anterior adductor scar small, seniilunar, 

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

 elevated or thickened radial ridge. The oldest Devonian species (M. cucullatus Goldf.) 

 has amorphous cardinals and a sniooth rounded shell (ßumegalodon). The Triassic 

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

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

 abundant 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 Lyc. {Pachymegalodon Gümb.). Like MegalodoUy but with 

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

 strong posterior lateral. Trias to Upper Jura. 



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

 adductor. Lias. 



Protodiceras Böhm. Lias. Dicerocardium Stoppani. Rhaetic. 



Megalomus Hall. Silurian ; North America. 



Family 28. Unionidae Fleming. 



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

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

 external ; lohes of the mantle generally united to form an anal siphon, hut the functional 

 hranchial siphon always incomplete helow ; foot compressed, keeled, large, rarely with a 

 feehle hyssus ; 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 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, 

 thougli 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 niore evidence is obtainable. 



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

 have sliown that considerable ditFerences exist as regards the soft parts, more especially the 



Fig. 699. 

 Unio stachei Neumayr. Pliocene (Congerian stage) ; Sibinj, Sla venia, p, 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 tliis System with 

 certainty to fossil Unionidae. 



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



^ Tausch, L. vo)i,_tJheT Conchodus, etc. Abhandl. geol. Keichsanstalt, 1892, vol. xvi. 

 2 JSTeumayr, M., Über die Herkunft der Unioniden. Sitzber. Akad. Wiss. Wien, 1889, xeviii. 

 •^ Wöhrmann, S. von, Über die systematische Stellung der Trigoniden und die Abstammung 

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



