SUBMYTILACEA. 



719 



Silurian. The Devonian genus Modiomorpha is closely related to 

 Modiolopsis, but the left valve has a single cuneiform tooth, which 

 fits into a corresponding cavity in the right valve. 



Family 2. Trigoxiid.f. — The members of this family are marine, 

 with a large bent foot, which does not secrete a bvssus, and with 

 free mantle-lobes. The shell (fig. 592) is equivalve, subtrigonal, 

 nacreous internally, with an external ligament placed behind the 

 beaks, and a simple pallial line. The hinge-teeth are few in number 

 (%• 59 1 ).- varying from one to three in each valve. The only living 

 genus in this family is Trigonia itself, but various extinct types may 

 be associated with this. 



In the genus Trigonia (figs. 591-593) the shell is thick, nacreous 

 internally, trigonal in shape, and very inequilateral, the beaks being 



mm 



Fig. 591. — Interior of the left 

 valve of Trigonia pectinata. 

 Recent, ff, Hinge-line, with 

 teeth and sockets ; a, Anterior 

 adductor impression ; a, Pos- 

 terior adductor impression. 



592. — Trigonia costata, from the 

 Jurassic rocks. 



directed backwards. The anterior side of the shell is rounded, and 

 the posterior side is produced and obliquely truncated, constituting 

 a special "area," which is often ornamented differently to the rest 

 of the shell, from which it is divided off by a more or less pro- 

 nounced ridge running from the beak to the hinder margin. The 

 so-called " escutcheon " is a smaller area, which is cut off from the 

 rest of the shell by a second oblique ridge running close to the 

 dorsal margin of the shell 

 behind the beaks. The 

 hinge of the right valve is 

 furnished with two divero-ins- 

 teeth, the faces of which are 

 strongly striated ; while that 

 of the left valve has a strone: 



o 



central tooth, striated on 



both sides, and two lateral, 



externally striated teeth. The surface of the shell is rarely almost 



smooth, the majority of species exhibiting a characteristic ornamen- 



m.r 



Fig. 593. — Trigonia scabra. Chalk. 



