G GENESIS OE THE AKIETID.E. 



of the first lateral lobes and the shortness of the ventral lobe. The aspect of 

 the second laterals in many species, and the gradation from these into the 

 auxiliary lobes, show that they retain the more primitive aspect of the earlier 

 forms in this part of the sutures. 



Ptychites of the Trias has sutures similar to those of Gymnites, and the 

 modified aspect of marginal lobes and saddles in both genera shows that, in 

 spite of a near approach or resemblance in the sutures to many Lytoceratinaa, 

 they cannot be considered as so nearly related to them as to Cyclolobus. 



Mojsisovics says that the evidence of genetic connection of Psiloceras planorbe 

 and Gymnites in'cultus rests alone upon the resemblances of the auxiliary lobes 

 and saddles, and that the resemblances in form only occur between the discoidal 

 Gymnites and the most involute Psiloceratites, the former being indeed much 

 more involute than the most involute of the Psiloceratites. The genus Halorites 

 of the Trias is regarded by Mojsisovics as the probable ancestor of the Arietidas. 



We cannot recognize that there are any very marked differences in the 

 amount of involution or form between Gym. incultus and Gym. Palmed when 

 compared with Psil. planorbe, and the resemblances of the sutures are exceed- 

 ingly close, especially when the species of Psiloceras of the Mediterranean 

 province are studied. The aspect of the shells in the three former are very 

 similar, while in the types of Halorites already cited by Mojsisovics, Hal. 

 Ramsaiieri, semiplicatus, decrescens, and semiglobosas, 1 they are very distinct. 

 The range of form in Halorites embraces highly sculptured shells, altogether 

 triassic in aspect. Neumayr's 2 and Warmer's 8 researches entirely confirm the 

 position here taken and show that Psiloceras possessed a series of involute 

 shells. Psiloceras and Gymnites, therefore, appear to be two parallel genera 

 of the same group, in each of which discoidal forms give rise to more involute 

 shells. Gym. incultus may be traced into the more involute Gym. Hmnboldti, and 

 the still more involute Gym. Creclneri. The adolescent young of Gym. Palmai, 

 Mojsis. 4 and incultus 6 show less involution than the adult, and we may confidently 

 expect that some correspondingly still less involute discoidal ancestral forms 

 will be found. Mojsisovics has not yet published his observations in full, and 

 his evidence is therefore not completed ; but, so far as we now know, the deriva- 

 tion of Psiloceras seems to have been from Gymnites as a common ancestor and 

 not from any forms of the Ceratitince like Halorites or its allies. 



Mojsisovics has said, that out of his group of Ammonites leiostraca the genus 

 Phylloceras alone persists and is but little changed in the Jura ; whereas the 

 Amm. trachyoslraca, or Ceratitinas, are more largely perpetuated, though much 

 changed, in the true Ammonitinaa. Our view differs, since we consider all 

 groups of the Trias to have been discontinued in the Jura except the Lytocera- 

 tinas. It is probable that a close affinity existed between Psiloceras and Gym- 

 nites, and the former is a modified Triassic survivor in the Lias ; but the 

 constant reappearance of the psiloceuan form in the young of undoubted Arietian 



1 Amm. Gattungen, Verhand. Geol. Reiclis., 1S79, No. 7. 



2 Unterst. Lias, Abhandl., Geol. Reich., YU. 8 Unt. Lias, Mojsis. et Neum , Beitr., III. 

 4 Med. Triaspiov., pi. lvii. fig. 2. 6 Ibid., pi. liv. fig. 3. 



