278 PALAEONTOLOGY OF NEW YORK. 



which are continued around the subcircular muscular area of the narrow um- 

 bonal cavity. The muscular scars consist of a narrow median or adductor, 

 enclosed by flabelliform diductors. 



In the brachial valve the area is linear, the delthyrium is progressively filled 

 by the growth of a callosity, which is often deeply grooved along the center, and 

 sometimes perforated in the line of division between the branches of the cardinal 

 process. The cardinal process consists of two sessile, diverging apophyses which 

 have broad, flat, striated surfaces of attachment, and are extended beyond the 

 hinge-line. The sockets are moderately deep ; the crural plates are usually not 

 sharply defined, but are continued in a curving line along the inner surface of 

 the valve, partially embracing a pair of broad, ovate muscular impressions 

 which are marked by arborescent ramifications ; recurving and again incurv- 

 ing, these ridges partially surround a pair of smaller muscular areas, lying in 

 front of the first. At the inner base of each branch of the cardinal process 

 there arises a low elevation or callosity, which, extending obliquely forward, 

 and uniting in the center, continues as a narrow median ridge dividing the 

 posterior pair of muscular impressions. This ridge sometimes terminates in a 

 point near the base of the first pair of impressions, and the second pair are 

 separated by a low, slender median septum, which sometimes apparently takes 

 its origin at this point, but which is in fact a continuation from the interrupted 

 posterior ridge, and extends for some distance over the pallial region. 



The muscular area, when its features are most distinctly retained, shows a 

 subdivision into the following scars : (a) A large posterior pair (the posterior 

 adductors), the surface of which is covered with arborescent ridges ; the ante- 

 rior portion of each of these scars is smoother than the rest, generally much 

 thickened and often extremely elevated at its outer margins. These may 

 be regarded as accessory elements of the posterior adductors. (6) An anterior 

 pair (anterior adductors), situated close together at about the center of the 

 valve. The position of these is generally well defined but their outline is fre- 

 quently obscure, (c) An elongate, narrow median scar, which is apparently 

 divided for its entire length by a faint ridge. In front of the muscular area 

 there are often a number of short protuberances on each side of the median 



