253 
Distribution of the Rhabdophora. 
the much more ancient family of the Dichograptida?, if its con¬ 
nexion with a compound form could not be satisfactorily 
proved. 
Hisinger’s Diplograptus (Prionotus ) pristis was equally 
fertile of errors. It has been identified with nearly all our 
commoner Diprionidse in turn, from the base of the Skiddaw 
to the horizon of the Wenlock shales. Portlock’s species 
Monograptus tenuis , Barrande’s species Monograptus Nilssoni ,, 
and Murchison’s Diplograptus foliaceus afford us instances of 
the same phenomenon. 
So generalized and defective were the original figures and 
descriptions of the graptolitic forms first detected, and so ob¬ 
scure or insignificant did their special characteristics appear 
to palaeontologists, that the foregoing list could be greatly 
extended. Even forms belonging to the most diverse genera 
were long confounded. The cautious and painstaking Bar- 
rande himself placed all his CUrnacograpti * in the genus 
Monograptus , supposing these diprionidian species to be scala- 
riform impressions of monoprionidian forms. Hall, again, 
placed the whole of the genera of the Dichograptidaj in the 
genus Monograptus f, on the ground that the simple and 
unilateral character of the polypary in the latter genus was 
incapable of demonstration. Retiolites \ and Diplograptus were 
long confounded; and the strikingly distinct genus Dicrano- 
graptus remained undistinguished. Perhaps the most remark¬ 
able confusion reigned in the bifid forms. Even as late as 
1873 the term Didymograptus was still employed by some 
of our greatest authorities so as to embrace forms belonging 
to the three groups Didymograptus , Dicellograptus , and Lep- 
tograptus , which are not only distinct genera, but are possibly 
the types of three distinct graptolitic families. 
(c) Previous Opinions. 
In the face of such formidable difficulties and misconceptions, 
geological and paleontological, it could not but happen that 
the generalizations of those who attempted to fix the range of 
the various forms of the Graptolitide in space and time were 
frequently wide of the truth. Looking back, however, over 
the history of the progress of our exact knowledge of these 
fossils from our present standpoint, it becomes clear that those 
errors — though of sufficient moment to shake the faith of the 
* Grapt. de Boheme, pi. ii. figs. 7, 8, 14, 15 j pi. iii. figs. 5, 6. 
t Grapt. of Quebec Group, p. 4 2 ; Twentieth Report of State Cabinet, 
pp. 236, 237. 
f Nicholson, Mon. Brit. Grapt. pp. 55, 63, &c. 
