1870.] 37 [Hyatt. 



DEROCERAS. 



Deroeeras Dudressieri. 



Amm. Dudressieri D'Orb., Terr. Jurass., Cepli., p. 325, pi. 103. 



From France, this species comes to us with the name of Amm. 

 hrevispina, and from England, as Amm. armatus or Birchii. With 

 none of these except Amm. armatus has it any close affinities. From 

 Amm. armatus it differs in the septa, besides having very different 

 young. The shell is strongly pilated and tuberculated and has the 

 planicostan abdomen very distinctly marked, whereas Deroeeras 

 armatus does not repeat this last feature so decidedly, being much 

 more cylindrical and smoother. The pilas are also closer together in 

 Deroeeras Dudressieri, the spines and pilas also being filled with solid, 

 shelly matter, instead of the spines alone, as in D. armatus. Oppel 

 has stated that he found Amm. Dudressieri of D'Orbigny in the Eng- 

 lish lower Lias, and this species is so closely similar in all respects to 

 D'Orbigny's figure of this species, that it seems to be the only one he 

 could have seen. D. confusum comes so near to the young of this 

 species that in external characteristics they seem to be nearly iden- 

 tical. 1 



The young is smooth for the first four whorls ; the pilas begin on 

 the fifth, but the tubercles are hardly visible until the last half of 

 the sixth. Soon after the pilae begin to appear, first as folds on 

 the sides, they stretch across the abdomen and form the planicos- 

 tan flexures. Though there are some slight differences between the 

 young of this species, on the fifth and earlier half of the sixth whorl, 

 and the typical planicosta, both in the shell and septa, they are 

 hardly sufficient to distinguish the two forms separated from the adult 

 whorls. On the seventh whorl the spines are very large but decrease 

 in prominence on the eighth, the pilaa approximating more. The 

 abdomen also becomes more elevated and rotund instead of rather flat- 

 tish, and the whole form approaches closely to what it is in Birchii. 

 The first three whorls have sides widely divergent ; these become 

 rounded on the fourth, flattened on the fifth and sixth, divergent on the 

 seventh, and rounded on the eighth. On the latter part of the tenth 

 whorl the tubercles entirely disappear, the pilas being reduced to mere 

 folds. The period at which these characteristics may be obtained or 



1 From this I of course exclude the form figured by Quenstedt as having a keel in 

 the young. 



