PLATE XXVII. 
1, 2. Orthocera Pyriformis. {Pear-shaped Orthocera.) Pyriform, tumid towards aperture, 
arched towards smaller end; smooth; section oval; siphunculus situate at one-third of diameter 
from edge. 
3. 0. Cordiformis. ( Heart-shaped Orthocera.) Ob-conical, heart-shaped; sides convex; aper¬ 
ture round ; surface smooth, plain; septa many; siphunculus nearly central; its tube inflated into 
globular form between each septum. 
4. 0. Paradoxicum. {Paradoxical Orthocera .) Lanceolate, triangular, flattened in front, 
edges projecting and producing gentle concavity; sides convex, somewhat dissimilar; aperture 
equilaterafly triangular, with sides rounded and hollowed in front; siphunculus central. 
5. 0. Angulare. {Angular Orthocera.) Sub-cylindrical, with a few, longitudinal sulci; septa 
situate very distant. 
6. 0. Gigantea. {Gigantic Orthocera.) Straight, tapering; finely striated; aperture oval, more 
than 8 in. in diameter; septa direct, deep, many; siphunculus situate a little distance from centre. 
7. 0. Inequiseptus. {Unequally-partitioned Orthocera.) Straight, tapering; septa unequal 
very distant in young shell ; section circular. 
8. 0. Reticulatus. {Reticulated Orthocera.) Elongated; septa distant; surface annulated ami 
reticulated with moniliform lines; sections circular. 
9. 0. Gesneri. {Gesner's Orthocera.) Curved, conical ; with about thirty longitudinal, close, 
acute, regular ridges and rounded sulci; siphunculus nearly marginal; section slightly ovate. 
( 31 ) 
