CLASS I 



PELECYPODA 



501 



none; lujament and resilium usually absent, an obsolete remnant of the resilium and 

 chondro])hore sometimes present in the left valve. (?) Carboniferous, Jura to Recent. 



Pholas Linn. (Fig. 833). Surface divided Ijy grooves into area.s whicli often Lave 

 diverse sculpture ; the adult often provided with accessory shelly plates, each of which 

 when seated in front of the beaks has been named 

 a " jirotoplax " ; when above the beaks, " mesoplax" ; 

 when behind the beaks between the valves, " nieta- 

 plax " ; and when between the valves ventrally 

 (Alartesia), " hypoplax." A calcareous septum, 

 secreted after the completion of tlie burrow, and 

 occujiying the pedal gape of the valves, is called 

 the "callum." The addition of these plates and 

 appendages during growth so changes the appear- 

 ance of the shell that old and young stages have 

 frequently been described as specifically or even 

 generically distinct. Typical Pholads date from 

 the Jura. Many subgenei'a have been named. 



Turnus Gabb (Fig. 834). Cretaceous. Martesia Leach (Fig. 835). 

 to Recent. Jouannetia Desm. Tertiary and Recent. Teredina Lam. 

 adult stage soldered together and to a thick adventive calcareous tube. 



Fig. 834. 



Turnus {Xylopha- 

 gella) elegantulus 

 Menk. Upper Cre- 

 taceous ; Idaho. 

 Enlarged (after 

 Meek). 



Fig. 835. 



Martesia ccmoidea 

 De.shayes. Eocene ; 

 Auvers, near Paris. 



Vi- 



Carboniferous 

 Valves in the 

 Eocene. 



Shell 



831-1. 



.1, Valves of the recent Teredo 

 norveglca Spengl ; inner and ontei- 

 views. B, Pallet of Xylotrya sp. 

 C, Pallet of Teredo sp. D, Casts 

 of boiings of Teredo tournali Leym. 

 Eocene ; Kressenberg, Bavaria. 



Family 46. Teredinidae Scjicchi. 



educed, equivalve, auriculate, widely galling, the valves a-pposited 

 ventrally only on the surface of a parietal tubercle ; adductor 

 scars unequal, the anterior marginal very small; pallial 

 line coincident loith the valve margins ; a styloid myophore 

 projecting from the cavity of the beaks; mantle secreting a 

 calcareous lining to the burrow; ixdlets variable in form, the 

 valves ivithout attached accessory shelly plates; area none; 

 hinge margin reflected, edentulous; ligament absent or 

 obsolete ; anterior adductor degenerate, attached on the 

 anterior edges of the valves, and covered only by the mantle ; 

 animal boring, chiefly in wood. Carboniferous (?) ; Jura to 

 Recent. 



Teredo Linn. (Fig. 836, A, C). Pallets simple, spatuli- 

 form. Juia to Recent. 



Xylotrya Leach (Fig. 836, B). Pallets articulated, 

 bipinnate. Tertiary and Recent. 



The name Teredolites Leymerie, has been projaosed for 

 the casts of borings of fossil Teredos (Fig. 836, D). The 

 problematical genus Polorthus Gabb, from the American 

 Cretaceous, has been referred to this family. The 

 Paleozoic species are known only by burrows, which are 



of 



;omewhat doubtful origin. 



Vertical Range of the Pelecypoda. 



Two small forms of bivalve shells, Fordilla and Modioloides, occurring in the 

 Lower Cambrian of New York State, have been doulitfuUy referred to Pelecypods, 

 but are more probably to Ije regarded as Branchiopod Crustaceans. Aside from these 

 fossils, whose moUuscan affinities must be considered as highly problematical, Pelecypods 



