ORDER IV TELOTREMATA 339 
Sub-equally biconvex shells with concentric growth lines extended into 
lamellae. Ventral umbo not prominent, incurved, usually concealing the 

Fie. 581. 
Meristina tumida, Dalin. sp. Silurian; Gottland. A, Shell of the natural size. JB, Interior of ventral valve- 
C, Hinge-line and median septum of dorsal valve. 
pedicle opening and deltidial plates. Teeth supported by dental lamellae. 
Hinge-plate of the dorsal valve perforated by a “visceral foramen.” The 
A 

Fic. 582. 
Athyris concentrica, von Buch sp. A, Shell with dorsal valve partly removed. JB, Interior of dorsal valve, 
14. C, D, Frontal and lateral aspect of spiralia (after Davidson). 
peculiar jugum of this genus is illustrated in Fig. 582, C, D. Devonian and 
Carboniferous. Distribution general. 
Actinoconchus, M‘Coy. Athyroids with very wide radially striate con- 
centric lamellae. Carboniferous ; Europe. 
Cleiothyris, King. Athyroids with concentric rows of flat spinules. Car- 
boniferous and Permian. Distribution general. 
Seminula, M‘Coy. Smooth-shelled Athy- 
roids. Dorsal hinge-plate very prominent. 
Carboniferous. Distribution general. 
Spirigerella, Waagen. Carboniferous; South 
America and India. 
Amphitomella, Bittner. Smooth - shelled Fic. 583. 
Athyroids with a double cardinal process and Mec factinella _ trigonella,  Schloth. sp. 
4 : 3 : Muschelkalk ; Recoaro, Italy. 1/;. 
median septa in each valve. Alpine Trias. 
Tetractinella, Bittner (Plicigera, Bittner), (Fig. 583). Athyroids with 
four corresponding ribs on each valve. Alpine Trias. 
Pentactinella, Bittner. Athyroids with five corresponding ribs on each 
valve. Alpine Trias. 
Anomactinella, Bittner. Athyroids with a number of angular alternating 
ribs towards the anterior margins. Alpine Trias. 

