EARLY DEVONIC HISTORY OF NEW YORK AND EASTERN NORTH AMERICA 



167 



The shells from the Grande Greve fauna which present such characters 

 as those specified are wholesome looking individuals of the aspect of Mega- 

 lanteris, averaging larger than specimens of either Beachia suessana 

 or Me gal. oval is and yet they have a predominant 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, 

 those of the Grande Greve shell being printed in italics. 



Beachia suessana 



Outline elongate 



Margins introverted deeply at side 

 and slightly in front 



Surface finely striated 



Shell punctate 



Cardinal plate composed of tivo cush- 

 ioned crural bases cemented medially. 

 Foramen nsically open except in old 

 stages. No cardinal process 



Ventral adductor scar shallow and 

 faintly defined 



No vascular markings 



Dorsal muscle scar extremely faint 



Megalanteris ovalis 



Outline siibcirctdar 



Margins introverted but slightly at 

 the cardinal shoulders 



Surface smooth 



Coarse internal striations interlock- 

 ing at front margins 



Inner shell layer punctate 

 Cardinal plate flat and thickened 

 bearing a stout cylindrical process 

 doubly grooved at the summit. Fora- 

 men lost 



Ventral adductor scar deep, long, 

 sharply divided 



Vascular markings 



Dorsal muscle scar well defined and 

 clearly divided 



We may fairly summarize the above by the statement that the Grande 

 Greve shells essentially agree with Megalanteris ovalis in all 

 respects save that which has been regarded as the basis of the generic dis- 

 tinction, namely the structure of the hinge plate. Hence the shells are to 

 be referred to Beachia rather than to Megalanteris. 



This statement however obscures with words the actual relations. If 

 we analyze the structural features in order of ontogenic values it is evident 

 that Meg. ovalis simply represents a greatly progressed condition of 

 which Beachia suessana is a primitive expression and the Grande 

 Greve shell an intermediate stage. So far as the last named is concerned 

 this condition is evinced in a usually greater thickening of the hinge plate 

 than prevails in Beachia, and a less strong development of the muscular 

 scars than in M e g . ovalis. 



