594 Forty- fifth Report on the State Museum. 



This deltidium is most conspicuously developed in early stages of 

 growth, when it has the form of a tube or sheath, which char- 

 acter becomes obliterated as maturity approaches, by the increase 

 in the size of the cardinal process, and the deltidial callosity 

 formed about its base. In adult shells the foramen has become 

 inclosed by the substance of the shell, its external opening being 

 an oblique groove in front of the apex of the valve, and its inner 

 aperture appearing in front of the pedicle-scar. Xot infre- 

 quently the passage is closed at maturity. The teeth are very 

 divergent and quite conspicuous, generally supported by lamellae 

 which are continued around the subcircular muscular area of the 

 narrow umbonal cavity. The muscular scars consist of a narrow 

 median or adductor, inclosed by flabelliform diductors. 



In the brachial valve the area is linear, the deltlryrium is pro- 

 gressively 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 aborescent ramifications; recurving and 

 again incurving, 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 Ioav 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 impres- 

 sions, and the second pair are separated by a low. slender median 

 septum, which sometimes apparently takes its origin a1 this point, 

 but which is in fact a continuation from the interrupted posterior 

 ridge, and extends for some distance over the pallia] reg on. 



The muscular area, when its features are most distinctly 

 retained, shows the subdivision into the following scars: (a) A 

 large posterior pair (the posterior adductors), the surface o\' 

 which is covered with arborescenl ridges; the anterior portion of 



14« 



