322 Systematic Paleontology 



rows, but often decayed in various degrees, leaving corresponding funnel- 

 shaped cavities, commonly elliptical in cross-section, wider than high, 

 very variable in size, the major axis 15 to 40 mm. and the minor 10 to 

 30 mm. ; armor thin, 2 to 5 cm., usually joined to the internal parts by 

 a clear line, but without measurable thickness, but sometimes very 

 irregularly so joined, and occasionally showing a thin libro-cambium 

 layer; woody zone 3 to 10 cm. thick, usually with two or three more or 

 less distinct rings, the outer or parenchymatous zone thicker and firmer 

 than the inner or fibrovascular zone; medulla usually homogeneous in 

 structure, elliptical, the major axis 8 to 17 cm., the minor 3 to 9 cm. 



This is historicaJly the most important species of Maryland Potomac 

 cycads, most of the original types found by Tyson belonging to it. It 

 is also the most abundant species, and was the first species of cycad 

 trunk from America to receive scientific mention. The plate is from an 

 original daguerrotype sent to Sir Wm. Dawson, and by him to Carruthers, 

 who mentioned it in a note to his paper On the Cycads from the Second- 

 ary Eocks of Great Britain {loc. cit.) 



This was discovered by Tyson {loc. cit.) about 1860 between Balti- 

 more and Washington, who collected in all perhaps 10 or 12 of these 

 trunks. These excited much interest at the time, but did not receive 

 scientific description for over 20 years, although Tyson sent pictures of 

 them to various geologists, both in this country and abroad, and pre- 

 sented specimens to Professors Dawson and Marsh, and perhaps others. 



The exact geological horizon in the Potomac Group has not been es- 

 tablished with certainty for any except this one trunk, and for this 

 reason the localities will not be given for the other species, since, as the 

 specimens do not occur in situ, the point where they eroded out or were 

 plowed up has little significance, since they have all come from the 

 same circumscribed belt. It is quite possible that they are all of Pa- 

 tuxent age, and may have been reworked in later, even Pleistocene, de- 

 posits. The exact age of the trunks is of little biologic significance, 

 since the frond impressions are present throughout the various forma- 

 tions of the Potomac Group, the absence* of petrified trunks being due 

 entirely to physical conditions of deposition. 



