230 PALEONTOLOGY OF NEW YORK. 



Gbnus LYCOPHORIA, Lahusen. 1885. 



PLATE LXIII. 



This name has been proposed for the Atrypa nucella, Dalman, a species not 

 unlike Porambonites in general external features. The valves are rotund and 

 have neither fold nor sinus, so that the anterior margin of contact is almost 

 straight or very gently sinuous. The beaks are full and closely incurved and 

 only the pedicle-valve appears to have retained a foramen, though the cardinal 

 area is present in both. The brachial valve bears a hinge-plate which is 

 recurved into the pedicle-cavity of the opposite valve and is produced into a 

 long, curved cardinal process, bifurcate at its extremity. The crural plates 

 are connected with the elevated margins of the four adductor impressions. In 

 the opposite valve the teeth are supported by divergent plates which extend 

 forwjird for about one-half the length of the shell and rest upon the bottom of 

 the valve. Externally the shell is smooth in the umbonal regions, but anteri- 

 orly is covered with low, rounded plications crossed by fine concentric lines. 



The systematic relations of this species are very interesting. It is associa- 

 ted with Porambonites in the lower Silurian faunas about St. Petersburg and 

 in Scandinavia, and its similarity to that genus in contour and, to a certain 

 extent, in details, is apparent. 



While Porambonites is strongly orthoid in the structure of its cardinal feat- 

 ures, and NoETLiNGiA possesses the simple linear cardinal process of Platystro- 

 PHiA, Atrypa nucella adds to these orthoid features the cardinal process of a 

 streptorhynchoid, like Triplegia and Mimulus, thus presenting another point of 

 tangency between these shells and the pentameroids ; or, more precisely, 

 another phase in the development from the comprehensive primordial stock 

 represented by Protorthis, Billinosella, etc., toward the full and typical 

 expression of Orthis, Orthothetes, Strophomena and Conchidium. 



