88 SMITHSONIAN INSTITUTION 



locate an exceptionally well preserved skull and both lower jaws of one 

 individual, and other portions of the skeleton of five additional indi- 

 viduals of the little zeuglodont, Zygorhiza, which was our chief objec- 

 tive on this trip. In many places remains of this little zeuglodont 

 are almost as numerous as its larger relative. Among the contem- 

 porary inhabitants of the same coastal seas were a large aquatic snake 

 and a large tortoise-like turtle. Fish vertebrae measuring two or more 

 inches in diameter, teeth of sharks, and the curiously ridged rod-like 

 beaks of an extinct sword fish {Cylindracanthus ornatus) are fre- 

 quently noticed in the washes. An abundance of rounded shells of 

 small sea-urchins (Schizaster armiger and Macropneustes mortoni), 

 a small scallop {Pecten per planus), a little oyster (Ostrca falco), 

 with a thin shell fancied to resemble a hawk, and a large nautilus 

 (Aturia alabamensis) , the chambers of which are locally known as 

 " goat's heads," occur in the " Zeuglodon " horizon. 



The yellowish or gray marl, in which these zeuglodonts occur, is 

 often hardened around the bones. From two to four hard ledges, a few 

 inches in thickness and separated by layers of this marl, are also gen- 

 erally present in the " Zeuglodon " horizon. These ledges are quite 

 resistent to the effects of erosion. Bones found in such ledges are 

 generally difficult to extricate on account of the hardness of the matrix 

 and the rather soft consistency of the bones themselves. 



The typical outcrop of the Jackson formation is located near Jack- 

 son, Mississippi, from which locality it takes its name. The exposures 

 of this formation in the prairie region of central Mississippi occupy 

 an irregular belt that ranges in width from about 6 miles near the Ala- 

 bama line in Clarke County to a maximum of more than 35 miles in 

 the region west of Jackson. The total thickness of the basal member, 

 consisting of quartz sand and glauconite, and the overlying clay mem- 

 ber is approximately 150 feet in eastern Clarke County. This forma- 

 tion increases in depth in a westerly direction and attains a thickness 

 of more than 600 feet in Yazoo County. Inasmuch as the " Zeuglo- 

 don " horizon is relatively thin and is situated near the base of the 

 upper clay member, which attains a total thickness of 70 feet in 

 Clarke County, one usually has to locate deeply eroded prairies to be 

 successful in a search for this fossil-bearing horizon. Several skele- 

 tons of the gigantic Basilosaurus were located along the eastern border 

 of Clarke County, but erosion and cultivation of the land had de- 

 stroyed whatever value they may have originally possessed for 

 paleontological study. Portions of two skeletons of the diminutive 

 zeuglodont, Zygorhiza, were excavated in this area. 



