452 MOLLUSCA. 



landinia (fig. 299), the two valves are not very dispropor- 

 tionately unequal, the shell is often more or less depressed, 

 and there is but a short mesial septum in the ventral valve 

 supporting a V-shaped chamber beneath the beak, whereas 

 in the dorsal valve there are only two short socket-plates. 

 Pentamerella, Ampliigenia, Gypidula, and Anastropkia are 

 other generic or sub-generic titles which have been proposed 

 for forms more or less closely allied to Pentamerus itself. 

 The first and second of these occur in the Devonian, but all 

 the above forms are characteristic of the Middle and Upper 

 Silurian. 



FAM. VI. STROPHOMENHLE. Animal unknown; some prob- 

 ably free others attached, during the whole or a portion 

 of their existence, by a muscular peduncle. No calcified 

 supports for the arms. Shell with a straight hinge-line and 

 a low triangular area in each valve. Shell-structure fibrous 

 or punctated. The Strophommidas are exclusively Palaeozoic, 

 and the principal genera are Orthis, Orthisina, Davidsonia, 

 Stropliomena, Streptorliynclms, and Leptcena. 



In the genus Orthis the valves are articulated by teeth 

 and sockets, and usually are more or less transversely oblong 

 (figs. 300-303). There is a straight hinge -line, generally 



Fig. 300. Orthis David- 

 soni; dorsal and side view. Fig. 301. Orthis porcata; dorsal and 



Silurian. side view. Silurian. 



shorter than the width of the shell. Each valve has a hinge- 

 area, notched in its centre by a triangular fissure through 

 which the fibres of the peduncle were transmitted. The 

 shell is often more or less flattened or depressed, and the 

 surface may be smooth, but is more commonly ornamented 

 with striae, or furnished with well-marked longitudinal ribs. 

 The species of the genus Orthis begin in the Cambrian, and 

 abound in the Silurian, Devonian, and Carboniferous periods, 



