ADESMACEA. 



745 



careous plate which fills up the anterior vacuity in the valves, and 

 in the production from the hinder end of the shell of cup-shaped 

 appendices which may coalesce to form a longer or shorter tube. 



In the genus Pholas, the shell is cylindrical or oval, and the front 

 portion of the valves is marked with conspicuous radiating ridges 

 or rows of spines. The valves are edentulous ; and there is no liga- 

 ment, or a rudimentary one. The pallial sinus is very deep, and the 

 dorsal margin of the shell is protected by accessory valves. The 

 Pholades inhabit burrows which they form for themselves in clay, 

 peat, or rock, and they are known in the fossil condition not only 

 by their shells but also by their burrows, the latter affording useful 

 indications of the existence of old shore-lines. Various extinct 

 forms are known from the Tertiary rocks, and the genus appears to 

 be represented in the Cretaceous and Jurassic deposits. 



In the recent genus Pholadidea the shell has a transverse furrow, and 

 the anterior vacuity of the valves becomes filled up with a callous plate. 

 In the genus, or sub-genus, Parapholas, again, the anterior aperture be- 

 comes similarly closed by a callous plate, but the valves show two ob- 

 lique furrows running from the beaks to the ventral margin (fig. 631). 



Fig. 631. — Parapholas mersa, viewed from one side and above. Cretaceous. 

 (After Stoliczka.) 



The species of Parapholas range from the Cretaceous rocks to the pres- 

 ent day. In the allied Martesia the anterior vacuity is also closed by a 

 callous plate, and the beaks are covered by a simple shield-like lamina. 

 The living species of Martesia burrow in wood, and the earliest fossil 

 forms have been detected in the Carboniferous rocks. The recent genus 

 Xylophaga also comprises wood-boring forms, and is represented in rocks 

 as ancient as the Jurassic and Cretaceous by allied types (Xylophagella). 

 Lastly, Fischer places in this family the Eocene genus Teredina, in which 

 there is a globular shell, the anterior vacuity of which is closed by a cal- 

 lous plate, while the beaks are covered by a dorsal plate, and the valves 

 are fused with a long calcareous tube, developed posteriorly. 



Family 2. Teredinid^e. — This family includes only the single 

 genus Teredo, the characters of which are, therefore, those of the 

 family. In Teredo, the shell is " globular, open in front and behind, 

 lodged at the inner extremity of a burrow partly or entirely lined by 

 shell ; valves three-lobed, concentrically striated, and with one trans- 

 verse furrow ; hinge-margins reflected in front, marked by the anterior 

 muscular impressions ; umbonal cavity with a long curved muscular 



