378 Systematic Paleontology 



Superfamily TEREBRATU LACEA 

 Family CENTRONELLID/E 



Genus RENSSEL/tRlA Hall' 



Individuals of tliis genus are prolific in the Upper Oriskany of Mary- 

 land, less abundant in the Becraft, apparently very rare in the Xew Scot- 

 land, and more common in the Keyser. They represent a progressive series 

 of variable or plastic species beginning with small forms and terminating 

 in giant forms of these early terabratuloids. When specimens are 

 assembled in quantity from all the horizons it will be seen that the species 

 intergrade, but if proper allowance is made for stratigraphic occurrence, 

 recognizable distinction will be found. 



Eensselasria sensu-stricto can be easily distinguished from Beachia by 

 the radial striations and the nearly angular inflected crenated margins of 

 the valves. The striations in Eenssela'ria are always pronounced, while 

 in Beachia they are only well developed along the line of junction of the 

 valves, making there a toothed edge, or along the edges of the rugosities. 

 Over the surface of the valves the striations are never distinct, and when 

 present are never sharply separated by deep grooves as in Eensselseria. 



R. mutabilis begins in very small individuals in the Keyser and attains 

 its specific characters in the Coeymaus. In New York, Hall had derived 

 his material from the New Scotland, but in Maryland the species is rare 

 at this horizon. In the Lower Oriskany the developmental sequence is con- 

 tinued by R. subglobosa and in the higher southern Oriskany by R. manj- 

 landica and R. circularis, but by 7?. ovoides and A', caijiiga in the northern 

 Upper Oriskany. 



Eenssel^eria mutabilis (Hall) 

 Plate LXVI, Figs. 5, 6 



Meganteris mutabilis Hall, 1857, Tenth Ann. Kept. N. Y. State Cab. Nat. 



Hist, p. 97. 

 Rensselwria mutabilis Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 254, pi. xlv, 



figs. 2a-2p, 1861. 



'■ For an extended description of this genus, see Hall and Clarke, 1893, Nat. 

 Hist. N. Y., Pal., vol. viii, pt. ii, pp. 255-260. 



