BARRANDE — SILURIAN CEPHALOPODA OF BOHEMIA. 



25 



Cyrtoceras heteroclytum, Barr. 



Fig. 9. 



proximate to Orthoceras, and form a transitional group between the 

 Fig. 8. <^wo genera. The mouth in nearly all 



my species of Cyrtoceras is analogous 

 to that of Orthoceras, i. e. it is not 

 contracted, but corresponds in shape 

 to the cross-section of the shell. Still, 

 I have met with a small number of 

 species in which the body-chamber is 

 swollen in the middle and again nar- 

 rows towards the mouth, such as C. he- 

 teroclytum (fig. 8 «, 6) ; nevertheless 

 the mouth preserves the same shape as 

 the transverse section of the shell. 

 Thus there is a transition from Cyrtoceras to Phragmoceras ; and 

 these genera also approximate in their style of curvature. 



7. Goinphoceras. — This genus differs from Ortlioceras chiefly in 

 the contraction of its mouth to a small aperture (fig. 9 «, V). All the 



species known to me are but slightly 

 elongate, and therein they approach the 

 second division of the Orthocerata above 

 described. Like these also the Gom- 

 phoceras has an inclination to curve, 

 especially in the chambered portion : 

 fig. 9 a shows one of the sides nearly 

 straight, and the other somewhatarched. 

 This analogy, however, does not extend 

 beyond the external form ; for in the 

 Bohemian Gomphocerata the position of 

 the siphuncle is very variable. Some- 

 times we find it in the centre, often between the centre and the convex 

 or dorsal border, sometimes between the centre and the ventral or 

 straight border. In respect of external ornamentation this genus 

 has but little variety, since most of the species are marked merely by 

 the lines of growth. Nevertheless they are easily distinguished by 

 the form of the mouth, the position of the siphuncle, &c. 



8. Phragmoceras. — For a long time I held a conviction that this 

 genus totally differed from Goinphoceras on account of the curvature 



and especially the constant 

 position of the siphuncle on 

 the concave or ventral bor- 

 der. Circumstances have 

 changed my views, for in 

 1851 I discovered, to my 

 great astonishment, &.Phrag- 

 moceras very well character- 

 ized by the contraction of its 

 mouth and the curvature of 

 its shell, but possessing a 

 dorsal siphuncle. Since then 

 I have obtained many spe- 

 cimens with this anomalous position of the siphuncle ; and, altbough 



Gomphoceras. 



Fig. 10 



Phragmoceras Brodcri/tii, Barr. 



