MOLLUSCA. LAMELLIBRANCHIATA. 149 



interior under the umbones is a long narrow plate for the 

 insertion of the pedal muscle. Tube long, calcareous, 

 sub-cylindrical, straight or curved, often with partitions. 

 Jurassic (? Carboniferous) to present day. 



Distribution of the Lamellibranchiata. 



All the Lamellibranchs are aquatic animals, and by far 

 the larger number are marine. The marine forms range 

 from the shore down to a depth of 2900 fathoms ; they 

 are most abundant in shallow water, at depths greater 

 than 500 fathoms they are scarce. 



The lamellibranchs appear first in the Tremadoc Beds, 

 and they gradually increase in importance, attaining their 

 maximum at the present day. Many of the genera have 

 a rather extended range in time. In the Palaeozoic 

 formations the Heteromyaria and the Homomyaria are 

 predominant, the former attaining thus early its greatest 

 development. In the Mesozoic, the Integripalliata reach 

 their maximum, the Monomyaria are also abundant, 

 whereas the Sinupalliata are represented by a few forms 

 only. In the Tertiary, the Monomyaria and Heteromyaria 

 are much less abundant, the prevailing forms belong- 

 ing to the Sinupalliata. 



The most important genera in the different systems 

 are as follows : 



Cambrian. Modiolopsis, Glyptarca, Ctenodonta. 



Ordovician. Ambonychia, Modiolopsis, Palaarca, Ctenodonta. 



Silurian. Avicula, Pterinea, Modiolopsis, Cardiola, Ctenodonta, 

 Grammy sia. 



Devonian. Pterinea, Cucullaa, Cardiola, Ctenodonta, Megalodon. 



Carboniferous. Aviculopecten, Posidonomya, Anthracosia, Cono- 

 cardium, Edmondia, Sanguinolites. 



Permian. Monotis, Bakevellia, Axinus. 



