MANTELLIA NIDIFORMIS. 157 
the Isle of Portland, are so fully described, that it will not 
be necessary to dwell upon them ; the structure and affini- 
ties of the fossil vegetables are the especial objects of our 
present inquiry. 
Manveuuia. Lign. 50, 51.—The fossil Cycadeze of the Isle 
of Portland were first described botanically by Dr. Buckland, 
(Geol. Trans. vol. ii. 2d Series,) under the name of Cyca- 
deoidea ; of which Memoir the account in Bd. p. 404, is an 
abstract. M. Ad. Brongniart, considering these plants as a 
peculiar type, referred them to a new genus, which he did me 
the honour to name Mantellia (Prod. Veg. Foss.). These stems 
Lien. 50. Silicified Trunk of MaANTELLIA NIDIFORMIs (Brongn.); 1 nat. 
(Cycadites megalophyllus. Bd.) 
Wealden. Isle of Portiand. 
a. Central mass of cellular tissue. 6. Circle of ligneous plates. c. Zone 
of cellular tissue. d. False-bark. 
or trunks are from one to two feet in height ; the circum- 
ference of the largest not exceeding three feet. The stem is 
