BARRANDE ON THE TRILOBITES OF BOHEMIA. 35 



compact mass, and almost without offering a trace of the rocks 

 which had before prevailed, and had till then excluded the limestone. 

 There must have been in Bohemia a long and unbroken continuance 

 of the action of causes which had elsewhere produced their effects 

 at intervals, schistose and siliceous deposits alternating with calcareous 

 ones, as in England, France, and elsewhere. 



From the apparent continuity of the beds in this basin, one would 

 be inclined at first to suppose that the subdivisions complicating the 

 geological scale might here be dispensed with. Palaeontological 

 facts, however, of which only the most striking are mentioned, 

 render it necessary to form three distinct groups. 



Group E. ( Lower Calcareous Group.) — 1 . Geological characters. 

 — The author states that he has not been able to discover hitherto 

 any want of conformability between the limestones of the group and 

 the beds on which they repose ; the passage being unbroken, so 

 that it is almost impossible to discover the line of separation. The 

 calcareous matter seems to have been gradually introduced, and 

 commences by the introduction of nodules of moderate size (from 

 half an inch to 16 inches diameter) and sparingly distributed; these 

 gradually increase in abundance but not in size ; they afterwards 

 become united and form irregular bands ; these then assume greater 

 regularity, alternating with the schists, and at length completely re- 

 place and exclude them. 



The colour of the lower calcareous bands is generally blackish, or 

 of a very decided grey. The beds are compact, and occasionally con- 

 tain veins of calc-spar and geodes of hyaline quartz. The siliceous 

 element is rare. The thickness of the whole group varies greatly, 

 but may be stated at from 30 to 120 yards. The thickness of the 

 separate beds varies from an inch or two up to about six feet. 



2. Palceontological characters. — The introduction of the calcareous 

 nodules corresponds with an important change in the fauna of this 

 period ; not a single species found in this newer group being identical 

 jvith those of the lower beds. 



The trilobites, far from disappearing, offer a much greater field of 

 variety. The genera represented differ from those found in (D), 

 except in the case of Chirurus, Phacops, and Odontopleura. The 

 dimensions, however, attained by these animals are much less con- 

 siderable in the limestones than in the lower beds. Notwithstand- 

 ing also the increased number of species, the trilobites cease to be 

 the predominant fossils in these newer deposits. 



On the other hand, the Cephalopoda, before scarcely represented, 

 here exhibit a prodigious development ; and the following genera 

 are met with, their relative abundance being in the order in which 

 their names are quoted. We have — 1. Cryptoceras, a genus formed 

 by the author to include certain hitherto unknown and very strange 

 forms ; 2. Gyroceras ; 3. Nautilus ; 4. Gomphoceras ; 5. Phragmo- 

 ceras ; 6. Lituites ; 7. Cyrtoceras ; 8. Orthoceras. 



Of these eight genera the six first-mentioned have most of them 

 only from two to four species, and specimens are rare; but the 

 Cyrtoceras exhibits about fifty distinct forms, and the Orthoceras 



