LAMELLIBRANCHIATA. 



225 



ceous series. The shells of this genus are inequivalve, with 

 radiating ribs or concentric furrows, and with prominent 

 beaks. The hinge-line is long and straight, with numerous 



Fig. \ig.Inoceramus sukatus. Gault (Cretaceous). 



Fig. 180. Inoceramus probl 



caitilage-pits. Some of the Inocerami attain a length of two 

 or three feet, and fragments of them are often found perfor- 

 ated by boring sponges. 



The last genus of the Aviculida is Pinna, in which the shell 

 is equivalve and wedge-shaped, and the beaks are placed 

 quite on the anterior side of the shell. The Pinnce seem to 

 have commenced in the Devonian, but their existence prior 

 to the Carboniferous period is a matter of some uncertainty. 

 Many species are known in the Secondary and Tertiary 

 Rocks, and the genus is well represented by living forms. 

 The sub-genus Trichites is exclusively Oolitic, and the shells 

 referred here sometimes attained an enormous size. 



FAM. 3. MYTILID^E. Shell equivalve ; umbones anterior; 

 hinge edentulous ; anterior muscular impression small, pos- 

 terior large. Shell attached by a byssus. Mantle-lobes united 

 between the siphonal apertures. Foot cylindrical, grooved, and 

 byssiferous. The chief fossil genera of the Mytilidtz are 

 MytihtS) Modiola, Lithodomus, Modiolopsis, and Orthonota. 



In the genus Mytilus are the true Mussels, in which the 

 shell is wedge-shaped, and the beaks terminal. Numerous 

 fossil forms are known, commencing in the Permian. The 

 Modiolce, or " Horse-mussels," have the beaks anterior, blunt, 

 not pointed, the hinge edentulous, and the shell oblong, 



