THE CEPHALOPODA 151 



Examples of some of the most highly organized 

 ammonoids (Asteroceras, Xipheroceras] , as well as of 

 degenerate forms (Spiroceras, Toxocems, Baculites] have 

 already been described. Examples of. the most primi- 

 tive forms will now be given. 



In the Upper Devonian shales of Biidesheim in the 

 Eifel district of Western Germany, there are found great 

 numbers of internal casts of small tightly-coiled cephal- 

 opod shells, in pyrites oxidized on the surface to a rich 

 brown colour. They rarely exceed 15 mm. in diameter, 

 but they are practically complete shells, since they show 

 the cast of the body-chamber as well as the septate 

 portion. They vary very much in relative thickness 

 and degree of involution, so that a number of different 

 species are distinguishable, but if the septal sutures are 

 examined they can at once be sorted into two series. 

 We choose an example of each to describe. 



4. Manticoceras [Gephyroceras] retrorsum has (in 

 the specimen shown in Fig. 42) a diameter of 15 mm. 



and its proportions are 







15: 53> 57> 20. 



The body-chamber, so far as preserved, is not quite 

 half the last whorl in length. The umbilical margin falls 

 very steeply, almost perpendicularly near the aperture, 

 but less steeply in the earlier-formed part of the last 

 whorl. The lateral and peripheral areas seem at first 

 sight to pass insensibly into one another, but very careful 

 examination reveals a very faint longitudinal depression 

 about 2^ mm. on either side of the middle line : this is 



