CLASS I 



PELECYPODA 



501 



none ; liijament and resilium usually ahsent, an obsolete remnant of the redlium and 

 chondrophore sometimes present in the left valve. (?) Carbon it'eroiis, Jura to ßecent. 



Pholas Linn. (Fig. 833). Surface divided by grooves iiito areas which ofteii have 

 diverse sculpture ; the adult ofteii provided with accessory shelly plates, eacli of which 

 when seated in front of the beaks has been named 

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

 when behind the beaks between the valves, " nieta- 

 plax " ; and when between the valves ventrally 

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

 secreted after the conipletion of the burrow, and 

 occupying the pedal gape of the valves, is called 

 the "callum." The addition of these plates and 

 appendages dnring growth so changes the appear- 

 ance of the shell that old and yoiing stages have 

 frequently been described as specifically or even 

 generically distinct. Typical Pholads date from 

 the Jura. Many subgenera have been named. 



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

 to Eecent. Jouannetia Desni. Tertiary and Recent. Teredina Lani. Valves in the 

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



Fig. 834. 



Turnus (Xylopha- 

 gella) ehgantulus 

 Meek. Upper Cre- 

 taceous ; Idalio. 

 Enlarged (after 

 Meek). 



Martesia conokka 

 Deshayes. Eocene ; 

 Auvers, near Paris. 



Vi. 



Shell 



Family 46. Teredinidae Scacclii. 



reduced, equivalve, auriculate, widely gaping, the valves apposited 

 ventrally only on the surface of a parietal tubercle ; adductor 

 scars unequal, thd anterior marginal very small ; pallial 

 line coincident with the valve margins ; a styloid myophore 

 projecting from the cavity of the beaks ; mantle secreting a 

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

 valves without 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, chießy in wood. Carboniferous (?) ; Jura to 

 Recent. 



Pallets simple, spatuli- 

 Pallets articulated, 



Fir,. 836. 



A, Valves of the recent Teredu 

 norvegica Spengl ; inner and outer 

 Views. B, Fallet of Xylotrya sp. 

 C, Fallet of Teredo sp. D, Gasts 

 of borings of Teredo tounmU Leym. 

 Eocene ; Kressenberg, Bavaria. 



Teredo Linn. (Fig. 836, A, G). 

 form. Jura to Recent. 



Xylotrya Leach (Fig. 836, B). 

 bipinnate. Tertiary and Recent. 



The name Teredolites Leymerie, has been proposed 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 somewhat doubtful origin. 



Vertical Range of the Pelecypoda. 



Two sniall forms of bi valve shells, Fordilla and Modioloides, occurring in the 

 Lower Canibrian of New York State, have been doubtfully referred to Pelecypods, 

 but are more probably to be regarded as Brauch iopod Crustaceans. Aside from these 

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



