FOSSILS OF THE O'OLITE. 



52. MIDDLE O'OLITE. This group, which is less complicated 

 than the preceding, at the lowest part consists of clay, called Oxford 

 clay, with layers of calcareous grit, and stratoid masses of lime- 

 stone. Above these are found sands, and limestones which are 

 more or less o'olitic, and often ferruginous. In this group we find 

 deposits of o'olitic iron, which had already appeared in the pre- 

 ceding series. It is very rich in fossils, particularly ammonites ; 

 and the Jlnanchy'les bicorda'tus (Jig. 97) is very common. 



Fig. 97. AnancJiy'tes bicorda'tus. 



AnancJty'tfS is a genus of the family of Echini'dese, or sea-urchins, some, 

 times vulgarly called sea-eggs, The family contains thirteen genera, which 

 are distinguished from each other by the form and size of the ambula'cra, 

 (alleys) the narrow longitudinal portions of the shell of the echinus or sea- 

 urchin, which are perforated with a number of small orifices, giving pas- 

 sage to tentacular suckers, and alternate with the broad tuberculate spine- 

 bearing portions (see Jig. 70) and ;ilso by the position of the vent, and of 

 the mouth. Figure 70, p. 54, exhibits the ambula'cra, between the tubercles 

 to which the spines are attached in living species. 



53. What especially characterizes the Oxford clays is the pre- 

 sence, often in abundance, of a new species of Gryphae'a (Jig. 98), 



ftg.QS.Gryphat'adilata'ta. Fig.QQ O'streaMa'rskii. Fig.lQl. Terebra'tulaimpre'asa. 



the O'strea Ma'rshii (Jig. 99), which already commenced in the 

 preceding group, a great number of different terebra'tula, among 



52. Of what floes the middle o'olite consist ? What fossils belong to itt 

 5.'*. How are the Oxford clays especially characterized ? 



