56 PALAEONTOLOGY 



Pectunculus. It is shaped like a cylinder slit open along 

 one radius, and yawning at the slit. The closing of the 

 valves tends to close this fissure, and the outer layers 

 are under tension while the centre is compressed. 



The interior of the shell is nacreous ; the wear and tear 

 of the outer layer in the region of the umbo often leads 

 to the exposure of the nacreous layer on the exterior. 



The hinge-teeth are quite different from those of 

 Pectunculus : they are very large and few in number, two 

 in each valve (the left posterior one bifid). Their surfaces 

 of contact are deeply grooved, as is also the posterior 

 surface of the left posterior socket (this is sometimes taken 

 as a third tooth, but it does not project beyond the median 

 plane as a tooth must do). The right anterior and the left 

 anterior and part of the bifid tooth are carried on a kind of 

 buttress (myophoric lamina) which also supports the anterior 

 adductor muscle. This muscle - impression is partly 

 crossed by a series of ridges (which probably give greater 

 " key " or grip to tbe muscle). The posterior adductor 

 is rather larger, and is placed nearly half-way between 

 the posterior end and the umbo, just behind the posterior 

 hinge-teeth. The pallial Jine has no sinus. The margins 

 are not crenulate. 



Thus Trigonia, Pectunculus, and Nucula are all three 

 equivalve, opisthogyral, and integripalliate, and Trigonia 

 and Nucula agree further in having a nacreous interior, 

 and in being very inequilateral (though in Trigonia it is 

 the anterior region, in Nucula the posterior, which is the 

 shorter). The opisthodetic ligament and hinge-teeth are 

 features in which Trigonia differs strikingly from both the 



