LAMELLIBRANCHIATA. 239 



In the Lantern-shells (Anatina) the shell is oblong, gaping 

 posteriorly, and having the beaks directed towards the posterior 

 side (fig. 200). The hinge of each valve carries a spoon- 

 shaped cartilage-process. The 

 Anatince are doubtfully repre- 

 sented in the Devonian, and 

 still more dubiously in the 

 Silurian Rocks. They occur, 

 however, abundantly in the 

 Secondary Rocks, and are 

 present in smaller numbers in 



the Tei'tiarieS. F'g- zo 1 - Pholadomya (equivalvis. 



The genus Pholadomya in- 

 cludes a large number of shells, which are equivalve, ob- 

 long, and gaping posteriorly (fig. 201). The shell is thin, 

 ventricose, and adorned with radiating ribs on the sides. 

 The ligament is external, and there is a large 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 repre- 

 sented at the present day. 



The genus Myacites has a gaping ventricose shell, with the 

 umbones directed anteriorly, and the ligament external. They 

 are known in the Palaeozoic period, commencing in the Silu- 

 rian ; and they are represented in the earlier portion of the 

 Secondary period; but they seem to have died out in the 

 Chalk. 



FAM. 20. GASTROCH^NID^E. 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 Gastrochczna and 

 Clavagella, the existence of Aspergillum in a fossil state being 

 doubtful. 



In Gastroch&na 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. 202) the shell is oblong, one of the valves 

 being free, whilst the other forms part of a more or less elon- 

 gated calcareous tube, which is often divided by a longitudinal 

 partition and terminates in tubular openings. The fossil 

 Clavagelltz commence in the Upper Greensand, and the genus 

 is represented by several living species. 



FAM. 21. PHOLADIDJE. Shell gaping at both ends, without 



