PTEROPODA. 



Trimerella, Billings, 1863. 



Shell allied to that of Oholus, from whicL. it differs in the pos- 

 Ression in the interior of each yalye of three longitudinal septa 

 of variable length, which support a horizontal or concave plate. 



Distributio?i, 2 species. Siluiian. Canada. 



Obolella, Billings, 1861. 



Etymology , diminutive of Oholus. 



Synonym, (?) Keyserlingia, Pander. 



Type, Obolella chromatica, Billings. 



*' Shell ovate, circular or subquadrate, convex or plano-con- 

 vex ; ventral valve mth 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 larael- 

 lose edges." 



"In general form these small shells somewhat resemble 

 Oholus, but the arrangement of the muscular impressions is 

 different. In Oholus the two central scars have their smaUei 

 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 PTEEOPODA.* 



Hermiceratites, Eichwald, 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. Ptussia. 



Saxterella, Billings, 1861. 



Dedicated to Mr. J. W. Salter, late Palaeontologist to the 

 Geological Survey of Groat Britain. 



Shell small, slender, conical, straight, consisting of many 



♦ See p 346. 



63 



