254 PALEONTOLOGY OF NEW YORK. 



vidual. With .age it became filled up by adventitious deposits. The crura are 

 long, straight, inclining upward or toward the opposite valve and are expanded 

 at their extremities into palmate processes. The muscular scars in this valve 

 are sharply developed, forming together an elongate adductor area, clearly 

 divisible into an attenuate anterior pair and a broader posterior pair. The 

 former have about twice the length of the latter and are marked by trans- 

 verse, fine, closely-set wrinkles ; the latter are radiately and coarsely striate. 

 From the posterior termination of this area to beneath the hinge-plate extends 

 a broad, smooth sinus, from which is given off a pair of strong lateral branches, 

 which ramify over the genital area in the umbonal region. Thus, except in the 

 character of its crural processes, the structure of the brachial valve and the 

 indications in regard to the composition of the muscular system are identical 

 with those of Rensselcuria ovoides. 



As already observed, the specific type of Amphigmia elongata varies little, if 

 at all, during the existence of the genus. A. elongata, var. curta, was described 

 by Meek and Worthen, from the beds at Jonesboro, 111., which have been 

 regarded as of the age of the Oriskany sandstone of New York, though con- 

 taining a number of Upper Helderberg types. Similarly, A. elongata has been 

 reported b}' Billings from the Oriskany fauna of Cayuga, Ontario, in which 

 there is even a larger representation of Upper Helderberg species. In the 

 New York faunas it appeared first in the Schoharie grit, where it is not com- 

 mon. In the Corniferous limestone it abounds in certain localities, especially 

 in the western part of the state, and with the close of the Upper Helderberg 

 it disappears. 



The occurrence of fine, large and typical examples of the species in the 

 lower Devonian sandstones of the rivers Maecuru and Curua, Province of 

 Para, Brazil, has been noted by Rathbun (loc. cit.). 



OEhlert has described as Amphigenia ? Bureaui, a large shell with smooth 

 exterior and small spondylium in the pedicle-valve, resting on the bottom of 

 the valve and not supported by a septum, as in A. elongata. This shell is from 

 the lower or middle Devonian of Montjean and Challones (Maine-et-Loire), 

 France. 



