442 AXEL A. OLSSON 



or fused together, and are so placed as to cover open spaces along the 

 margins of the primary valves, and over the anterior, the anterior ventral 

 or pedal gap thus protecting the soft tissues within. Both the ligament 

 and the hinge teeth are absent or much modified. The umbonal margins 

 are inrolled over the beaks forming a rounded umbonal reflection which 

 may be appressed closely to the surface underneath or it may he widely 

 separated from it leaving an open or a multiseptate space. The walls 

 of the shell are usually thin and fragile; they may form a single uninter- 

 rupted surface or the disk may be divided into smaller areas by one or two 

 deep sulci which radiate from the beak, the areas so set apart differing 

 greatly in size, shape and sculpture; typically, the anterior area has a sculp- 

 ture of coarse, sharp, radial costae, while the posterior area is usually smooth 

 or marked with concentrics only. There is a slender apophysis in the um- 

 bonal cavity of each valve in some species; it is lacking in others. 



The pholads are typically marine species, although some forms have 

 become adapted to moderately brackish waters. In many species, the soft 

 parts cannot be retracted fully within the shell, the long, leathery siphons 

 remaining extended (see Plate 78, figure 2c). In the subfamily, Pholadinae, 

 the shell is similar at all stages of growth and the anterior gap, if one is 

 present, remains open. In the idealized sketch, (Plate 78, figure 1), copied 

 after Turner, 1954, the external surface of the right valve of a typical pholad 

 is shown, its sculpture, and the position of the accessory plates. 



In the most recent review of the family Pholadidae, that of Turner,^® 

 four subfamilies are recognized as follows: 



I. A slender, armlike process or apophysis is present in the umbonal 

 cavity. 



A. The anterior-ventral or pedal gap is open, not closed or covered by an 

 accessory plate or callum at any stage. External surface not divided 

 into separate sections, its sculpture fairly uniform or gradational over 

 the whole disk and usually consists of sharply scabrous radial riblets. 

 Animal not capable of complete retraction within its shell. 



Subfamily Pholadinae 



B. The pedal or anterior-ventral gap is covered by an accessory plate 

 (callum) in the adult stage. External surface divided into distinct 

 areas by one or two deep radial sulci. Sculpture dissimilar in the separate 

 areas. Animal contained within its shell. 



Subfamily Martesiinae 



II. No internal apophyses. 



C. Anterior gap covered completely by the callum in the adult, open in 

 the juvenile stages; other accessory plates may also be present. Surface 

 divided by an umbonal ventral sulcus into two distinct areas. Animal 

 contained within its shell. 



Subfamily J ouannetiinae 



D. Anterior gap is open at all stages of growth. Valves teredo-like. Animal 

 capable of complete retraction within its shell. 



Subfamily Xylophaginae 



l6Ruth Turner, 1954. Johnsonia, vol. 3, part 1, No. 33; part 2, No. 34. 



