(iKOLOGlCAI^ MIOMOIRS. 



millem. in length. This family thus presents its greatest develop- 

 ment as to size at this the earliest period of its existence. Fifthly, 

 there also occur various fragments of large Crustaceans, which I 

 cannot as yet refer to any known genus or even family. 



Pteropoda. — The Class of Pteropods in the Silurians of Bohemia 

 affords a somewhat large number of forms, belonging to two genera, 

 Conularia and Theca (Ptigiunculus) ; of the former I enumerate about 

 1 7 species, and of the latter 20 species. It is very difficult to obtain 

 perfect specimens of Conularia, particularly of the larger species oc- 

 curring in the schists. I have, however, succeeded in getting suffi- 

 cient to show the specific characters. The figure of a fragment of C 

 grandis occupies the whole of one of my plates. I have found, more- 

 over, that the surface-ornament of the shell is subject to considerable 

 variations in passing from the young to the adult state. In the case 

 of C. proteica, portions of which are very well preserved in the lime- 

 stones of my Upper Division, very many species might easily be made 

 out by any one studjang singly the several individuals found even in one 

 locality, much more specimens from different localities. Appearances 

 resulting from the difference of the inner and outer layers of the 

 shells may also easily mislead. The genus Pugiuncidiis was so 

 named by me in 1 847 (Jahrb. p. 554), not being then acquainted with 

 the name Theca, established for it by Sowerby and Morris a short time 

 previously*, and which therefore has the priority. Since then this 

 form has been recognized as an apparently characteristic fossil in 

 most of the Silurian regions. I have recognized it in the Primordial 

 Fauna of Bohemia, as well as in that of the United States. It occurs 

 also in the Second Silurian Fauna of Bohemia, England, Portugal, 

 France, the United States, New Zealand, &c. It is continued also 

 into the Third Fauna; and IMM. Sandberger have described two spe- 

 cies of this genus in the Devonian Fauna of the Rhenish district. 



I may here remark that I have discovered an operculum [Deckel] 

 with this shell, in the first instance with T. striahda; and I have 

 hereby been enabled to recognise the true nature of certain similar 

 but disconnected bodies which I had met with. These covers of the 

 Theca are easily known by their triangular outline f, corresponding 

 to the transverse section of the Theca itself, the form of which is that 

 of a more or less elongated [three-sided] pyramid. 



Cephalopoda. — The Ccphalopods, as a Class, are, as I have else- 

 where said, the richest in species of all the Classes of animals that 

 have formerly existed in the Silurian basin of Bohemia. Although I 

 am not quite prepared with the descriptions of all the species, I be- 

 lieve their number may be estimated at from 280 to 290 ; whilst of 

 Trilobites I have described only 253 species. The large number of 

 the species of these Cephalopods is in strong contrast with the small 

 number of their genera. I know of but ten genera in the Bohemian 

 basin. 



[* Strelecki's New South Wales, 1845, p. 289.— Traxsl.] 

 [t Figures of the Theca and its operculum (together with illustrations of various 

 Cephalopodous shells, are given in a plate in the Jahrbuch, accompanying the ori- 

 ginal. — Traxsl.] 



