SOME NEW DEVON I C FOSSILS 



249 



The shells under present consideration which afford such 

 characters as those specified, are wholesome looking individuals of 

 the aspect of Megalanteris, averaging larger than specimens of 

 either B. suessana or M. o v a 1 i s and yet they have a pre- 

 dominant similarity to the latter. They combine however in most 

 instructive manner the characters of both these species and genera, 

 and we endeavor to express this relation by comparison in tabulated 

 form with the distinctive characters of each. 



BEACHIA SUESSANA 



Outline elongate 



MEGALANTERIS OVALIS 



Outline sub circular 



Margins introverted deeply at 

 side and slightly in front 



Surface finely striated 



Shell punctate 



Cardinal plate composed of two 

 cushioned crural bases ce- 

 mented medially. Foramen 

 usually open except in old 

 stages. No cardinal process 



Ventral adductor scar shallow 

 and faintly defined 



No vascular markings 



Margins introverted but slightly 

 at the cardinal shoulders 



Surface smooth 



Coarse internal striations inter- 

 locking at front margins 



Inner shell layer punctate 



Cardinal plate flat and thickened 

 bearing a stout cylindrical 

 process doubly grooved at the 

 summit. Foramen lost 



Ventral adductor scar deep, 

 long, sharply divided 



Vascular markings 



Dorsal muscle scar extremely 

 faint 



Dorsal muscle scar well defined 

 and clearly divided 



We may fairly summarize the above by the statement that the 

 shells under consideration essentially agree with M. o v a 1 i s in all 

 respects save that which has been regarded as the basis of the 

 generic distinction, namely the structure of the hinge plate. Hence 

 the shells are to be referred to Beachia rather than to Megalan- 

 teris. 



This statement however obscures with words the actual rela- 

 tions. If we analyze the structural features in order of ontogenic 

 values it is evident that M. o v a 1 i s simply represents a greatly 

 progressed condition of which Beachia suessana is a prim- 



