58 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. I. 



usually occur, and which invest them with a peculiar interest. 

 These vegetables are from one to two feet in height, the 

 circumference of the largest not exceeding three feet. The 

 stem is sub-cylindrical, without a distinct central axis, and the 

 surface is scored with rhomboidal scars, which are widest in 

 the horizontal direction. There are two species, distinguishable 

 by the form of the stems and the size 

 of the cicatrices of the petioles. In one 

 the stem is short and spheroidal, and 

 the leaf-scars are relatively broader. 

 (M. nidiformis) : this species is named 

 "crow's nest" by the workmen, who 

 believe these plants to be nests built 

 by crows in the trees with which 

 they are collocated, and that the trees 

 and nests have become petrified toge- 

 . ther - ^e other species ( M. cylindrica) 

 isle of Portland, ffl sub-cylindrical, and relatively higher 

 than the former, and the cicatrices of 



the petioles are much smaller, indicating a more delicate 

 foliage. The fruit of these plants is unknown ; one cone has 

 been found which it is supposed belonged to the Mantellia, 

 or some allied species ; it is figured in the Fossil Flora of 

 Great Britain as Zamta crassa. 1 



FOSSIL WOOD AND TREES OF AUSTRALIA. Table-case a. 

 In the recesses of the east windows, and in the table-cases 

 beneath, there are many choice specimens of the wood, and 

 two portions of very large trunks of coniferse, from Van 

 Diemen's Land and New Holland. This fossil wood is partly 

 calcified, and partly silicified ; some portions being very earthy 

 and friable, and effervescing strongly with acid, while other 

 parts of the same stem are converted into chalcedony and 

 semi-opal ; in both states the organic structure may be 

 detected by the aid of the microscope. 



The trees from which the specimens brought to England 



1 " Medals of Creation," p. 160, lign. 38. 



The fossil plants of the Isle of Portland are admirably described and 



illustrated by Dr. Buckland, in Bridgewater Essay, p. 497, and pi. 60, 



v nr S e 8 P ecies described in the text under the names assigned to them 



f D Bkf niart ' are the Cycadites nwatoptyttus and C. microphyllus 



