326 SPIRirERTD^. 



and form two large curves facing the lateral portions of the 

 A^alve ; on approaching the front they form four or five convo- 

 lutions, which become smaller to the terminal coil, which faces the 

 middle of the lateral portions of the shell ; near the front the 

 primar}'" lamellge give off two processes which converge and 

 extend between the spiral coils in an upward and backward 

 direction ; after becoming united towards the middle of the 

 shell, they are again prolonged in the shape of a single lamella, 

 ■which proceeds upwards for a little distance with its extremity 

 directed towards the hinge-plate. In the interior of the ventral 

 valve a mesial groove extends to about the middle of the shell, and 

 on either side, running parallel with the hinge-line, are two broad, 

 rounded projections, at the outer extremity of which is situated 

 the articulating tooth ; below these are the elevated muscular 

 scars. 



HiNDELLA, Davidson, 1882. 



Dintr. — H. umbonella, Billings (cxxxvii, 15). Palasozoic ; 

 Anticosti. 



Shell elongate ovate ; spiral cones with their apices directed 

 towards the lateral margins of the shell ; about six coils in each 

 spiral ; two principal stems of the spiral cones attached to the 

 hinge-plate, and after extending a little way into the interior of 

 the shell between the spirals, suddenly bent backwards towards 

 the hinge ; they then form a broad, rounded curve, facing the 

 ^bottom of the dorsal valve, the curve being very gentle, so that 

 the two primary lamellae on the dorsal side seem almost like 

 parallel lines ; when the primary lamella reach the fi'ont they 

 give off a semicircular band or loop having a projection or spike- 

 like process at the top of it ; this loop is directed upwards 

 towards the beak, and is almost immeoliately behind the two 

 primar}' lamellae on the dorsal side of the spirals ; the loop is 

 therefore internal to the spirals. 



TJncites, Defrance, 1826. 



Distr. — Fossil. Devonian ; Europe. U. gryphus, Schloth. 

 (cxxxviii, 31, 32). 



Shell impunctate ; oval, biconvex, with a long incurved beak; 

 foramen apical, closed at an early age ; deltidium large, concave ; 

 spiral processes directed outwards ; no hinge-area. 



The large, concave deltidium of TJncites so much resembles 

 the channel formed by the dental plates of Pentamerus, that 

 Dalman mistook the shell for a member of that genus. The 

 discovery of internal spires, by Professor Bey rich, shows that 

 it only differs from Retzia in being impunctate and destitute of 

 hinge-area. Some of the specimens have corresponding depres- 

 sions in the sides of the valves, forming pouches which do not 

 communicate with the interior. 



