138 A Guide to the Zoological Collections in the 



end, and the usually long and more acute portion of the 

 shell is the posterior end. The free edge of the shell cor- 

 responds with the ventral surface of the animal, while over 

 the back of the animal the valves interlock along w^hat is 

 known as the "hinge-line''. 



This interlocking is effected by means of "teeth", of 

 which there are typically three, or three groups, one (or a 

 group) immediately below the umbo known as " cardinal 

 teeth", and two (or two groups) of " lateral teeth " placed 

 respectively before and behind the cardinal. Any or all of 

 these, however, may be absent. The hinge becomes a 

 spring-hinge by the action of certain elastic ligaments the 

 principal one of which passes across, from valve to valve, 

 behind the umbo, usually on the outside. When, by the 

 muscular efforts of the animal, the shell is closed, the hinge- 

 ligament is put upon the stretch ; and when, therefore, the 

 muscular effort is relaxed, the tension on the ligament is 

 relieved, and the shell springs open : so that in repose the 

 shell always gapes. 



On the inside of each valve, which is almost always 

 polished, we usually find certain constant impressions, 

 namely the pits of attachment of the adductor muscles, and 

 the line of attachment of the mantle — " pallial-line ". 

 These should be studied in the series of prepared speci- 

 mens in Case 172 A. 



Notice that in one type the palliaUline has an unbroken 

 sweep from the anterior to the posterior adductor, while in 

 another type the sweep is broken by a deep bay — the 

 " pallial sinus " — which lies below the impression of the 

 posterior adductor and marks the situation of the retractor 

 muscles of the siphons. 



The Lamellibranch " foot " is usually tongue-shaped, 

 or hatchet-shaped, and of some size. It can be protruded 

 between the valves of the shell and used for creeping. In 

 a few cases — e.g., Niicula — it forms a broad " sole " for 

 this purpose. In some forms — e.g., the Cockles — it is 

 curved and is used as a jumping-pole, the Cockle being able 



