506 LAM ELLIBR ANCHI AT A. 



entire pallial line. Sanguinolaria, Semele, and Scrobicularia 

 are represented both in the Tertiary rocks and in our own 

 seas ; while Capsula and Mesodesma, with living forms, appear 

 as early as the Secondary period. 



FAM. 17. SOLENID.E. Shell elongated, gaping at both ends; 

 ligament external ; hinge-teeth usually 2-3. Siphons short 

 and united (in the long-shelled genera), or longer and partly 

 separate (in the genera with shorter shells). Foot very large 

 and powerful. Gills prolonged into the branchial siphon. 

 This family is of small geological importance. In the true 

 Eazor-shells (Solen) the shell is greatly elongated, with ter- 

 minal beaks, and gaping at both ends. The genus is said to 

 occur in the Carboniferous, but well-marked types do not 

 appear till the Tertiary rocks are reached. Solenopsis, of the 

 Carboniferous rocks, is believed to be allied to Solen, from 

 which it differs in its prominent beaks and closed anterior 

 end, but its true affinities are not certain. Cultellus (fig. 

 371, i>) has a shorter shell than Solen, which gapes at both 

 ends and has the beaks placed a little in front of the centre, 

 and supported by an oblique rib. It commences in the 

 Cretaceous, as does the genus Solecurtus, in which the beaks 

 are also sub-central. 



FAM. 18. MYACID^E. Shell gaping posteriorly. Mantle 

 almost entirely closed ; siphons united, partly or wholly re- 



rig. 374. Portion of the hinge 

 Fig. 373. Mya truncate. Post- Pliocene of Mya arenaria, showing the car- 



and Recent. tilage-process. 



tractile. Foot very small. The more important genera of 



this family are Mya, Corbula, Thetis, Panopcea, and Saxicava. 



In the Gapers (Mya) the shell is oblong, inequivalve, and 



gaping at both ends. The left valve is the smallest, and 



