3376 



THE MOLLUSKS OR SHELLFISH 



and Tertiary rocks. The valves of Trigonia are beautifully pearly within, equal, 

 radiately ribbed, with an external ligament, and a few strong striated divergent 

 hinge teeth. The umbones are inclined posteriorly a very unusual feature in 



bivalves. The foot of the animal is large 

 and powerful, used in crawling and leap- 

 ing, and without a byssus. In some of 

 the Jurassic rocks of Weymouth trigonias 

 form a bed several feet in thickness. 

 Mussels (family Mytilidce} are such well- 

 known shells that a description is unneces- 

 sary. They are found all over the world; 

 one very large species from the shores of 

 California sometimes reaching fully nine 

 inches in length. They anchor them- 

 selves by a byssus, but also have the 

 power of moving from place to place, by 

 casting off the byssus, extending the foot 

 in the direction they determine to proceed, 

 and attaching a byssal thread, which 

 supports the animal while the foot is 

 again extended and another thread spun. 

 This process is repeated again and again, 

 and thus progress is made. The structure 

 of the animal may be understood by 



observing the accompanying illustration, where a represents the edge of the mantle; 

 b, the foot; r, the byssus; d, e< the foot muscles; _/, the mouth; g, the labial palpi; 



COMMON MUSSEL, OPENED TO SHOW THE VA- 

 RIOUS ORGANS (natural size). 



DATE SHEIKS IN THE ROCK. 

 (Natural size.) 



