22 



GEOLOGICAL MEMOIRS. 



doubt once existed there. In the next stage of growth (fig. 2 b) one 

 whorl is complete. As the shell increases, there is added gradually 

 a second and a third whorl. 



In N. Bohemicus and N. Sternbergi, when the third whorl is at- 

 tained, the diameter reaches 20-25 centim. ; and this appears to be 

 the full growth of the shell. In the third species which afforded me 

 the opportunity of studying this development, the diameter of the 

 shell does not surpass 7-8 centim. 



It is a singular fact, that we must recur to the Silurian rocks and 

 to the relics of the earliest species of Nautilus to discover the deve- 

 lopmental forms of a genus, which presents itself in so many species 

 throughout all the geological formations, and which even exists in 

 our present seas. 



Among the other cephalopodous genera of Bohemia, Trochoceras 

 is the only one that has afforded me e.nbryonic individuals, but I 

 have not yet been able to collect a perfect series of the transitional 

 forms from the embryo to the adult. Such also is the case with 

 Orthoceras ; sometimes we meet with very fine and almost needle- 

 like casts of the shell, which, however, have no decided specific cha- 

 racters about them. 



2. Lituites. — In comparison with the northern species, the Bo- 

 hemian Lituites are all very small, and their straight portion has 

 barely a width of a few centimetres in the tangential direction. Two 



Fig. 3. '^^ these, in which I found the mouth well preserved, 

 exhibit an in-bending of the two opposite lateral 

 margins, as in Phragmoceras and Gomphoceras. 

 We see here that the aperture consists of the same 

 parts as those which some years since I pointed 

 out in these two genera, namely the main opening, 

 fig. 3, a, the siphuncle, c, and the fissure, d, which 

 Mouth of a Lituite. unites them. As I have not seen the northern 

 Lituites, I do not know whether they possess a similar conformation, 

 or, like Orthoceras, have an uncontracted aperture. 



3. Gyroceras. — Four or five species of Gyroceras occur in this 

 district ; and nearly all are pro^dded with lateral processes of the 



Fig. 4. shell, which mark the stages of its 



periodical growth. The species that 

 best exhibits this ornamentation is 

 my G. mirum, fig. 4, a, b. It is 

 very difficult to obtain specimens 

 with the processes of the shell per- 

 fect ; but I have succeeded in 

 working out a great part of the 

 last whorl. It is not, however, on 

 account of the ornament that I have 

 named this species as above ; but 

 the mouth of the shell ai)pears to me very wonderful, being neither 

 round nor elliptical, as in other alhed forms, but half-closed by a 

 bending-back of the shell on itself. Looking at the mouth of this 

 shell, one might think that for half of its extent it had been closed 



Gyroceras mirum, Barrande. 



