PTEKOPODA. 



Trimerella, Billings.. 1863. 



Shell allied to that of Obohis, 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. 



Etymology, diminutive of Obolas. 



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 is 

 different. In Obolas 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 PTEEOPODA.* 



Hermiceratttes, 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. Eussia. 



Saxterella, Billings, 1861. 



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

 Geological Survey of Great Britain. 



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

 * See p. 346. 



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