DAVIDSON — rAL^:ONTOLOGICAL NOTES ON TUE BKACIUOPODA. 101 
The following are the gcucra into which the families under dcscrij)- 
tion have been provisionally divided* : — 
Geological K:mge. 
Genera or Sub genera, Author, Date, and 
O 
a Tyiiical Species. 
c 
.S 
c 
Cm 
c 
« 
6 
a 
o 
O 
.n 
1 
a 
o. 
Ha 

c 
PoRAMBONiTES. Pander. 1830 . 
y 
P. cequirostvis. 
OiiTHis. Dahiian. 1827 
X 
X 
X 
? 
0. riis'ica. 
Orthesina. D'Orb. 1849 . . . 
X 
0. anoniala. 
Streptorhynchus. King. 1850 
1 
X 
X 
X 
STROPIIOMiiNIDjE < 
S. pelargoiiatus. 
Strophomexa. Rafinesque. 1S20 
S. planumhona. 
X 
X 
X 
. 
Lept.ena. Dahiiaii. 1827 . . . 
X 
X 
X 
7 
X 
S. transversuUs. 
Tkopidoleptus. Hall. 1857 . . 
X 
• 
T. carinatus. 
Strophodonta. Hall. 1S52 . . 
X 
S. depressa. 
■ 
Chonetes. Fischer. 1837 . . ■ 
X 
X 
X 
X 
C. sarcinuluta, or C. Prattii. 
Stkophalosia. King. 1844 . . 
X 
X 
y. 
• 
PRODUCTID^ . 
& Goldfussii. 
AuLosTEGES. Helmcrsen. 1847 . 
A. Wangcnheimi. 
X 
Proddcta. Sowerby. 1814 . . 
? 
X 
X 
X 
• 
I 
P. r/iyaiitea. 
The generality of palaeontologists of the present day appear to be 
agreed as to the propriety of forming two families out of the genera 
or sub-genera above enumerated ; but I am beginning to fear that 
the characters by which some are distinguished will turn out to be of 
less importance than was at one time imagined. 
It has been stated that external spines were peculiar to the Fro- 
* FamUy names are derived fiom those in the typical genera ; and, as in tlie 
present instance Strophomena and Productus were established first, it behoves us 
to admit them in preference to others subsequently introduced. NatiuaHsts have 
not hitherto agreed as to what should constitute a genus or sub-genus ; and as 
some even entirely object to the term " sub-genus," I have not made any such dis- 
tinctions in the table here given ; but I have endeavoured to aiTange the names 
according to their more probable affinities. When treating of the StropJwmenidce, 
we will endeavoiu- carefully to compare the different "genera" or "sub-genera" 
with each other ; as the differential characters do not appear to me to have been 
in all cases satisfactorily estabhshed. 
