256 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



DESCRIPTION OF GENERA AND SPECIES 

 NEVADIA, new genus 



Dorsal shield broad, ovate. Cephalon large, semicircular in out- 

 line, about one-third the length of the dorsal shield ; genal angles 

 extended into spines ; facial sutures rudimentary or in a condition 

 of symphysis ; eyes crescentic, with ridges uniting them with the 

 anterior lobe oi the glabella ; glabella elongate, with a relatively small 

 anterior lobe and three posterior transverse lobes ; strong occipital 

 ring. 



Thorax with twent}'-eight segments ; body of pleurcie nearly 

 straight ; pleural furrow broad and parallel to the transverse axis of 

 the jilcune ; pleura; terminating in long, curved spines that are much 

 shorter on the posterior eleven segments in the type species which 

 are without a distinct, furrowed pleural lobe. 



Pygidium small, without pleural lol)es and transverse furrows. 



Surface minutely granular and witli irregular network of fine, 

 irregular, anastomosing ridges. 



Genotype. — Ne^'odia 2ueeksi, new species. 



The generic name is given after the State of Nevada, in which the 

 specimens were found. 



Stratigraphic range. — Lower Cambrian : Silver Peak group where 

 the type species ranges through a band of arenaceous shale and 

 quartzitic sandstone 222 feet in thickness. In the Barrel Spring 

 section [VValcott, 1908, p. 189, 12 of section] the species was placed 

 under the genus Holmia. 



Geographic distribution. — Sixteen miles south and 10 miles north- 

 west of Silver Peak, Esmeralda County, southwestern Nevada. 



Observations. — Nevadia is probably the most primitive form of the 

 Mesonacida?. The strong ridge uniting the eye lobe and the frontal 

 lobe of the glabella in the adult is a marked feature of the young 

 of Elliptocepliala asaplwides [pi. 24, figs. 3, 6, 7; pi. 25, figs, q, 10, 

 11] ; OlencUus fmiioiifi |pl. 37. figs. 9, 10, 11] ; and, as a case of 

 reversion, in Oleiiellits thoinpsoiii [pis. 34, 35] and similar forms of 

 OlencUus from the upper portion of the Lower Cambrian terrane. 



Nevadia appears to be the more primitive type and it occurs much 

 deeper down in the Cambrian section than Mesoitacis ver)iiontana 

 and FJlipfocephala asaplioidcs. 



The elongate thorax of many segments ; spinose extensions of the 

 pleura? ; narrow ing of the pleural lobes and their absence on the ten 

 posterior segments ; and the very small, simple pygidium without 



