454 Geological Distribution of the Rhabdophora. 
only demonstrated the erroneousness of many previous iden¬ 
tifications, hut has resulted in freeing many forms hitherto 
undescribed, and thus greatly restricting the supposed extended 
geological range of the earlier and less perfectly described 
species. 
(e) Classification. 
Classification of the Rhabdophora. — Since the publication 
of the memoir last referred to, many new facts bearing upon 
the inter-relationships of these fossils have been detected, but 
none of sufficient moment to justify us in modifying to any 
material extent the scheme then proposed. It was pointed 
out at that time that our available evidence was sufficient to 
lead us to hope that the monoprionidian forms would be found 
to be grouped according to their natural affinities. This 
opinion is distinctly supported by our new data. The arrange¬ 
ment of the diprionidian or petaloid genera, on the other 
hand, was confessedly temporary and provisional. Here 
subsequent discoveries show most distinctly that the view 
then advocated of the possible separation of these forms into 
two main natural groups, by characters founded upon the 
presence or absence of the sicula and the single or double 
character of the coenosarcal canal, is no longer tenable. This 
is demonstrated by the fact that the sicula has been detected 
in the Lasiograptidae; while several typical Uiplograptidae 
have been found to possess a single coenosarcal canal, as in 
Retiolites proper. 
The following scheme will be adhered to in the present 
paper: — 
Table of the Families and Genera of the Rhabdophora. 
Fam. I. MoxooKAPTiDiE. Gen. : Monograptus, Gein. ; Cyrtograptus , 
Carr. ; Rastntes, Barr. 
Fam. II. Leftograptid je. Gen.: Leptograptus, Lapw. ; Pleurograptus, 
Nich. ; Ccenograptus , Hall ; Azygograptus, Nich. & Lapw. ; 
Amphigr aphis, Lapw. 
Fam. III. Dichograptidje. Gen. : Didymograptus, M‘Coy ; Tetragraptm, 
Salt. ; Dichograptus, Salt. ; Loganograptus, Hall ; Tem.no- 
graptus, Nich. ; Schizograptus , Nicli. ; Ctenograptus , Nich. ; 
Clematograptus , Hopk. ; &c. &c. 
Fam. IV. Die ra n o gi:a pt iu je . Gen.: Dicranograptus , Hall; Dicello- 
graptus, Hopk. 
Fam. V. Diplograptiiee. Gen. : Diplograptus, M‘Coy ; Climacograptus, 
Hall; Cephalograptus, Ilopk. 
Fam. VI. LASiOGRAPTiDiE. Gen. : Lasiograptm, Lapw. ; Glossograptus, 
Emmons ; Retiograptus, Hall ; &c. 
Fam. VII. Bktiolitid.e. Gen. : Retiolites, Barr.; Clathrograptus, Lapw.; 
Trigonograptus, Hall ; & c. 
Fam. VIII. Phyxlograptid.*. Gen. : Phyllograptus, Hall. 
