THE LAMELLIBRANCHIA 



53 



tension like the external ligament of Pectunculus, and it is 

 its resiliency (or tendency to recover its shape when the 

 pressure is removed) that causes the valves to open. 

 Nucula is isomyarian and integripalliate ; its valve- 

 margins are slightly crenulate. An important difference 

 from Pectunculus is that the inner shell-layer is nacreous 

 (pearly), that is to say it is composed of thin oblique 

 laminae of calcite bound together by organic matter 



ant.adJ. 



FIG. 13. NUCULA. 



Left-hand figure, internal view of right valve of N. margaritacea, Lamarch, 

 Eocene, (x 2. ) (After Deshayes.) Right-hand figures, dorsal view 

 (above) and right side view of internal cast of N, pectinata, J. Soweiby, 

 Albian (Gault). (Natural size.) (After Woods.) 



(conchiolin), and the reflection of light from the 

 surfaces of such thin laminae results in interference and 

 consequent iridescent colours. 



3. Trigonia is a genus very abundantly represented by 

 well-preserved specimens in the Jurassic system, to a less 

 extent in the Cretaceous, and very rare in later formations, 

 though still surviving in Australian seas. We will choose 

 the Upper Jurassic species Trigonia bronni (Fig. 14) for 



