510 



LAM ELLIBRANCHI AT A. 



pallial sinus. The species of Pholadomya are very numerous 

 in the Secondary rocks, where they attain their maximum. 

 They are much reduced in number in the Tertiaries, and are 

 barely represented at the present day. 



FAM. 20. GASTKOCILENID^;. Shell equivalve, gaping, with 

 thin edentulous valves, sometimes cemented to a calcareous 

 tube. Mantle-margins thick in front, united, with a small 

 pedal aperture. Siphons very long, united. Foot finger- 

 shaped. The members of the Gastrochcenidce burrow in mud 

 or stone, and the only two fossil genera are Gastroclicena and 

 Clavagella, the existence of Aspergillum in a fossil state being- 

 doubtful. 



In G-astrochcena the shell is wedge-shaped, gaping in front 

 and closed behind. The fossil species commence in the In- 

 ferior Oolite, and the genus 

 is represented at the present 

 day. In Clavagella (fig. 380) 

 the shell is oblong, one of the 

 valves being free, whilst the 

 other forms part of a more 

 or less elongated calcareous 

 tube, which is often divided 

 by a longitudinal partition 

 and terminates in tubular 

 openings. The fossil Clava- 

 gellce commence in the Upper 

 Greensand, and the genus is represented by several living- 

 species. 



FAM. 21. PHOLADID^;. Shell gaping at both ends, without 

 hinge or ligament, often with accessory valves. Animal club- 

 shaped or worm-like, with a short truncated foot. Mantle 

 closed in front; siphons long, united to near their extremities. 

 In the genus PJiolas the shell is cylindrical or oval, the 

 valves are edentulous, and there is no ligament or a rudimen- 

 tary 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. Many fossil species of the genus are 

 known, commencing in the Jurassic rocks. The living genus 



Fig. 380. Clavagella crelacea. Chalk. 



