28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



The muscle field of the pedicle valve is generally small in all species 

 but it is also deeply excavated into the shell. The individual scars are 

 usually strongly impressed. The diductor scars are small for such 

 large shells, are somewhat rectangular in form, and surround the 

 moderately large adductor patch. The adjustor scars are small and are 

 located just anterior to the front edge of the dental lamellae. Posterior 

 to the main part of the muscle field and within the delthyrial cavity the 

 floor of the valve is considerably thickened. Here two small scars, the 

 accessory diductors, are located. 



The genital area is small and is situated on the sloping sides of the 

 shell just anterior to the dental plates. The teeth are small, corrugated, 

 and are supported by thin dental plates that are nearly obliterated in 

 some specimens by thickening of the inside of the shell and filling of 

 the umbonal cavities. 



A prominent feature of Basiliola is the strong development of the 

 pallial markings. One main pair of pallial trunks, the vascula media, 

 originates between the diductor and adjustor muscles. A subsidiary 

 pair of trunks, the vascula genitalia, originates at the same place but 

 extends posterolaterally to surround the genital area. The vascula 

 media extend slightly anterolateral^ to just beyond midvalve where 

 they branch. The main vascula media then extend slightly antero- 

 medially to terminate on the outside of the tongue. The other branches 

 at midvalve extend laterally where they divide. One branch swings 

 posteriorly near the valve margin to die out just before reaching the 

 teeth. The other branch extends anterolaterally. Short vascula ter- 

 minalia are given off from the outside anterior part of the vascula 

 media and their lateral branch, the vascula arcuaria. 



The cardinalia of the brachial valve are characterized by the wide 

 and flat outer hinge plates to which are attached concave scimitarlike 

 falcifer crura. These are concave inward and are blunt at their distal 

 extremity. The crural blades are slightly oblique or nearly vertical as 

 viewed from the posteroventral side. The distal edge of the crura 

 facing the pedicle valve are usually finely serrate. 



No median septum is present but in some specimens the elongated 

 adductor field is divided medially by a faint myophragm. The adductor 

 scars are small and elongated. The posterior pair is much smaller 

 than the anterior pair. The genital area is small like that of the pedicle 

 valve and is surrounded anteriorly and laterally by vascula genitalia 

 that connect with the posterior end of the vascula media. The major 

 trunks in the brachial valve are like those of the pedicle valve. The 

 vascula media originate at the outer ends of the adductor field and 



