PTEROPODA. 
TRIMERELLA, Billings, 1863. 
Shell ified to that of Obolus, from which it differs in the pos-~ 
session in the interior of each valve of three longitudinal septa 
of variable length, which support a horizontal or concave plate. 
Distribution, 2 species. Silurian. Canada. 
OBOLELLA, Billings, 1861. 
Etymology, diminutive of Obolus. 
Synonym, (?) Keyserlingia, Pander. 
Type, Obolella chromatica, Billings. 
“« Shell ovate, circular or subquadrate, convex or plano-con- 
vex; ventral valve with a false area, which is sometimes minute, 
and usually grooved for the passage of the peduncle; dorsal 
valve either with or without an area; muscular scars in the 
ventral valve, four; one pair in front of the beak near the 
middle, or in the upper half of the shell, and the others situated 
one on each side near the cardinal.edge; shell calcareous; sur- 
face concentrically striated, sometimes with thin extended lamel- 
lose edges.” 
‘In general form these small shells somewhat resemble 
Obolus, but the arrangement of the muscular impressions ig 
different. In Obolus the two central scars have their smaller 
extremities directed downwards, converging towards each other - 
but in this genus the arrangement is exactly the reverse.”— 
(Billings. ) 
Distribution, 12 species. Lower Silurian. United States, 
Canada, England, Spain. 
CLASS PTEROPODA.* 
HERMICERATITES, Hichwald, 1840. 
Shell cylindrical or semi-cylindrical, elongated, straight, with 
a dark brown corneous epidermis, furnished with a straight, 
median siphuncle, which does not traverse any chambers. 
Fossil, 3 species. Middle Silurian. Russia. 
SALTERELLA, Billings, 1861. 
Dedicated to Mr. J. W. Salter, late Palzeontologist to the 
Geological Survey of Great Britain. 
Shell small, slender, conical, straight, consisting of many 
* See p 346, 
63 
