94 
MEETINGS OF SECTIONS. 
GEOLOGICAL SECTION. 
Thursday, November 29th (postponed from 22nd inst.) — Mr. A. 
Leipner in the chair. 
Mr. C. F. Ravis exhibited some specimens of crystals of quartz, and 
the President gave a short explanation of the probable mode of 
their formation, &c. 
Mr. W. Sanders, F.R.S., F.G.S., President of the Society and of the 
Section, then delivered an address upon Fossil Fishes. After stating that 
the mode of classifying fishes varied with the object proposed to be 
elucidated by the lecturer, he presented to the Society a table of fossil fishes 
adapted to show their relation to Geological epochs. The primary divi- 
sions separated the Cartilaginous from the Osseous. The first of these com- 
prised the great majority of the fossil, especially of those in the Palaeozoic 
strata, while almost all the species of recent fish belonged to the Osseous 
division. There were three orders of Cartilaginous fishes. The first, the 
Plagiostomi, had the mouth transverse across the under part of the head. 
The gills had five apertures, and the ventral fins were abdominal ; the 
tail was supposed to be heterocercal. Here these characteristic terms were 
explained, and drawings and specimens of fishes with heterocercal and homo- 
cercal tails were shown. This order contained four families : 1. The 
Cestraciontidae, containing Ptenacanthus, Psammodus, &c. ; 2. The 
Hybodontidse, containing Ocodus and Hybodus ; 3. The Squalidas or 
Shark-like fish ; and 4. The Raiidse, including Myliobates and others. The 
second order, the Holocephali, had gills with one aperture, fins with a 
spine, and abdominal ventrals. The one family of this order, the Chimse- 
roidse, were remarkable for possessing only one tooth in each side of each 
jaw. The Edaphodus was shown as an example. These two orders had 
a Placoid exo- skeleton. The third order of fish were covered with Ganoid 
scales. The four primary divisions, adopted by Agassiz, into Placoid, 
Ganoid, Ctenoid, and Cycloid, were noticed, and the terms explained. 
Having given a general account of Ganoid fishes, and of the three orders 
of the Osseous tribes, the speaker presented a more particular account of 
various Genera belonging to the two orders of the Placoid. It was then 
announced that the subject would be resumed on a future evening, and a 
description would be given of the Ganoid Families of Fish. 
