yi6 



LAMELLIBRANCHIATA. 



or slightly curved, and the hinge is essentially similar to that of Nucula, 

 except that the ligament is external ; but there exists a long internal 

 septum, which extends from below the beak towards the anterior muscu- 

 lar impression. The genera Cleidophorus (Ordovician and Silurian) and 

 Redonia (Cambrian and Ordovician) appear to be related to Cucullella, 



Fig. 586. — Megalomus compressus. a, Side view of the cast of the shell, natural size; b, The 

 same viewed from above : ad, Cast of the adductor impression ; p, Pallial line. (Original.) 



since both possess a vertical internal plate, which commences in front of 

 the beaks, and is continued downwards behind the anterior adductor, a 

 deep slit being left in the cast of the shell by its removal. 



Also in the neighbourhood of Nucula may be placed the genus 

 Tellinomya ( = Ctenodonta, Salter), though this type shows decided 



Fig. 588. — Tellinomya contracta. 

 Ordovician. a, Interior of right 

 valve ; b, Exterior of the same. 



Fig. 587. — Nucula bivirgata. Gault. 



affinities with Pedunculus among the Arcidce. In Tellinomya (figs. 

 583, b, and 588) the posterior side of the shell may or may not be 

 the shortest, and the ligament is external, and is not placed, as in 



