BARRANDE SILURIAN CEPHALOPODA OF BOHEMIA. 



23 



5. 



by a septum, the direction of which is symmetrical and inverse to 

 that taken by the septum at the base of the body-chamber. On 

 first examining these specimens, I was inchned to think that half 

 the aperture was closed by a loosened septum ; but further observa- 

 tions in eight or ten individuals showed that it could not be an acci- 

 dental condition ; and, lastly, I found a specimen in which the whole 

 circumference of the mouth could be traced with certainty. This 

 semiclosure of the orifice of the shell in Gyroceras appears therefore 

 to be analogous to the contraction of the mouth in Fhrcujmoceras, 

 Gomphoceras, and Lituites, above mentioned ; but it is peculiar, in 

 that it is not the lateral margins that are bent towards each other, 

 as in these genera, but only the inner (under) margin is pressed 

 back. 



4. Trochoceras. — I have before mentioned (loc. cit. p. 7) that 

 some few turreted species appear to form transitions between this genus 

 and Lituites and Gyroceras. I must, however, add, 

 that there is always a certain absence of symmetry 

 sufficient to distinguish the new type, and on which 

 it depends. So also with some other species, the shells 

 of which do not form a complete whorl, and which 

 without this [want of?] symmetry would readily be 

 referred to Cyrtoceras. The position of the siphuncle 

 in Trochoceras is very variable, — sometimes dorsal, 

 sometimes central, and sometimes intermediate. Most 

 of the Bohemian species have strongly ornamented 

 shells; in contrast to the Cyrtocerata, which are mostly 

 inornate. 



5. Orthoceras. — This genus supphes nearly half 

 the Silurian species of Cephalopods in Bohemia. The 

 species of Orthoceras, however, are 

 characterized with difficulty. I hope, 

 however, to conquer this by means of 

 the numerous specimens I have col- 

 lected, by which I can recognize the 

 chief elements of several specificforms, 

 such as the circumference of the 

 mouth of the shell, the body-cham- 

 ber, the air-chambers, the siphuncle, 

 the shell, and its ornamentation. 

 Without this it would be easy indeed 

 to confound very distinct species, frag- 

 ments of which in rock-specimens 

 (especially the chambered portions), 

 and destitute of the outer shell, pre- 

 sent very similar features. Thus we 

 still have grouped under the name 

 of 0. regulare species which would probably prove very distinct, were 

 we acquainted with all their characters. ^ , ^ .7 



I will not now enter upon the classification of the Orthoeerata ; 

 but I mav remark that the species belonging to my Ihird Stage may 



E 2 



Fig. 6. 



Orthoceras. 

 Elongate form. 



