THE LAMELLIBRANCHIA 57 



other genera. This type of hinge is described as schizodont. 

 According to the method of notation to be explained 



immediately, its formula is 



4. Cyrena semistriata is an abundant fossil in the 

 Oligocene beds of the Isle of Wight. The shape is sub- 

 trigonal, the umbo being about one-third the total length 

 back from the anterior end ; the anterior border is rounded, 

 the posterior part of the dorsal border sloping obliquely 

 back to a narrow rounded posterior end. (Apart from 



FIG. 15. CYRENA SP. RECENT. 



Interior of left valve. (Natural size.) Teeth white, sockets black. 

 p.s., Pallial sinus ; tig., ligament. (Original.) 



this shape, the remainder of the description will apply 

 in general to any other species of Cyrena, such as that 

 shown in Fig. 15.) The shell is equivalve. The outer 

 surface is smooth, with concentric growth-striations. 

 The ligament is opisthodetic and short. Internally, the 

 shell is isomyarian, and there is a very slight pallial sinus, 

 so that it may be described as feebly sinu-palUate. The 

 hinge-teeth are carried on a well-defined hinge-plate 

 and fall into three distinct sets (heterodont) : just under 

 the umbo are a series of short, more or less vertical 



