6 GENESIS OF THE ARIETID. 
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 Lytoceratina, 
| 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 Gymuites meultus 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 Arietide. I. 
We cannot recognize that there are any very marked differences in the | 
amount of involution or form between Gym. incultus and Gym. Palnai when 
| compared with Psi. 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, Had. 
Ramsaueri, semiplicatus, decrescens, and semiglobosus,' they are very distinct. 
The range of form in Halorites embraces highly sculptured shells, altogether 
triassic in aspect. Neumayr’s” and Wihner’s® 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. incullus may be traced into the more involute Gym. EHumboldti, and 
the still more involute Gym. Crednert. The adolescent young of Gym. Palmai, 
Mojsis.* and ieu/tus° 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 Ceratitine 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. trachyostraca, or Ceratitinss, are more largely perpetuated, though much 
changed, in the true Ammonitine. Our view differs, since we consider all 
groups of the T'rias to have been discontinued in the Jura except the Lytocera- 
tine. 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 psiloceran form in the young of undoubted Arietian 
1 Amm. Gattungen, Verhand. Geol. Reichs., 1879, No. 7. 
I 2 Unterst. Lias, Abhandl., Geol. Reich., VII. 8 Unt. Lias, Mojsis. et Neum., Beitr., III. 
i 4 Med. Triasprov., pl. lvii. fig. 2. 5 Thid., pl. liv. fig. 3. 
