LAMELLIBRANCHIATA. 



237 



FAM. 1 6. TELLINID^E. Shell free, usually equivalve and 

 closed; cardinal teeth t\vo at most, laterals i-i, sometimes 

 wanting. Ligament on the shortest side of the shell, some- 

 times internal. Mantle widely open in front. Siphons long 

 and slender ; foot tongue-shaped, compressed. Pallial sinus 

 very large. The chief fossil genera are Tellina, Psammobia, 

 and Donax. In Tellina the shell is very slightly in equivalve 

 (fig. 196) with a prominent external ligament. More than 

 a hundred fossil species 

 are known, dating from 

 the Oolitic period ; but 

 the genus has attained its 

 maximum at the present 

 day. In Psammobia the 

 shell is oblong, compres- 

 sed, and slightly gaping at 

 both ends ; whilst in Donax 

 the shell is wedge-shaped, the front rounded and produced, 

 the posterior side short. Both genera commence in the 

 Eocene Tertiary, and are represented by numerous species at 

 the present day. 



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 



Fig. 196. Tellina proximo,, right valve. 

 Post-Pliocene. 



Fig. iq-j.Myatruncata, Post-Pliocene 

 and Recent. 



Fig. 198. Portion of the hinge of 

 Mya areiiaria, showing the cartilage- 

 process. 



partly separate (in the genera with shorter shells). Foot 

 very large and powerful. Gills prolonged into the bran- 

 chial siphon. This family is of small geological importance. 

 The Razor-shells (Solen) are represented by a few Tertiary 

 forms, commencing in the Eocene ; and the genera Cul- 

 telhis and Solecurtus commence their existence in the Creta- 

 ceous Rocks. 



FAM. 1 8. MYACID.E. Shell gaping posteriorly. Mantle 

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



