LOWER PALAEOZOIC FAUNAS 14$ 



The two last-named series may possibly occur in the 

 Ordovician, but Lower Palaeozoic forms are proble- 

 matical and rare. Two genera, Orthodontiscus and 

 Allodesma^ are sometimes classed with the Teleo- 

 desmacea. They were probably Schizodont Priono- 

 desmacea having possibly ancestral relations with the 

 more elaborate group. Prionodesmacea were the chief 

 bivalves of the Palaeozoic ; their prominence in early 

 faunas was due rather to the feeble development of the 

 other two subclasses than to intrinsic abundance. In 

 actual numbers Mesozoic forms exceeded many-fold 

 those of the earlier era. An obscure series of Priono- 

 desmaceans with unhinged valves (Palaeoconcha), 

 ranging from the Silurian to the Holocene, is repre- 

 sented in the former period by a few forms, of which 

 Cardiola is a familiar fossil of the Wenlock and Ludlow 

 stages. The Taxodonts, which include forms whose 

 primitive qualities can be assessed by dissection, 

 appeared in the Ordovician. The two genera of chief 

 interest are Nucula and Leda> both of which have 

 persisted from the Silurian to the present day with 

 remarkably little change. They clearly represent the 

 " Lingula" type of evolution wherein early stocks are 

 perpetuated by almost static morphogeny. The most 

 important Schizodonts (a series that occurred in extreme 

 profusion in the Mesozoic) were the radicals of the 

 Pteriidae. Pterinea is common in the Upper Silurian, 

 and can be found in the Ordovician ; while Rhombopteria, 

 claimed as the true morphogenetic ancestor of the 

 family, was of Silurian date. Isodonts are represented 

 by early forms of Pterineopecten in the Silurian, but are 

 rare. The Dysodonts, so familiar under the name of 

 " Mussels " at the present day, were perhaps the 

 commonest series of Lower Palaeozoic Pelecypoda. 

 Modiolopsis is not uncommon in the Ordovician, and 

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