OPERCTJLA OF AMMONITES. 26 T 



quarters of a whorl, usually attaining a greater length. Apty- 

 chus ? Lobular line not very complicated, understood in the 

 sense of a reductive change. Siphonal lobe largest, external 

 saddle much developed and broad, laterals one-pointed, very 

 small in the geologically younger forms. 



The genus Simoceras begins in the upper part of the middle 

 Jurassic, with the group Sim. sulcatum, anceps, Greppini, 

 Fraasi, Rehmanni, which stand very close to typical Peri- 

 sphinctes, differing from them only in somewhat more developed 

 constrictions, the appearance of tubercles on the ribs and the 

 presence of an external furrow, so that it is in the highest degree 

 probable that both genera spring from a common root, especially 

 as Perisphinctes is inclined in a high degree to develop an 

 external furrow. These stand very close to forms from the 

 lower and middle parts of the upper Jurassic, as Sim. contortum, 

 Neum., and Agrigentinum, Gem. Gradual^ a change of such 

 a kind steps in, as to replace the originally present divided ribs 

 with more and more simple ones until the first are entirely replaced, 

 whilst at the same time the ribs on the body-chamber separate 

 more and more, and become strongly swollen. Out of these the 

 extremely developed, strange species of the Tithon are evolved, 

 for which the genus was originally established, with in part very 

 prominent, in part rudimentary sculpture, decidedly reduced 

 lobular markings, and with the external lobe of the aperture 

 bent upwards. 



The highest development is attained by Simoceras in the 

 Tithon, where a great diversity of forms are developed ; but 

 the genus already- begins to die out in the upper Tithon. The 

 geologically oldest forms are distributed in the Mediterranean 

 as well as in middle European Jurassic, more prominently in the 

 latter. The younger types are almost entirely confined to the 

 Mediterranean province, and appear northwards in quite isolated, 

 extremely rare species (Sim. Eandenense and Doublieri). 



Twenty-six species. 

 SIMOCERAS JOORAENSIS, Waagen. T. 112, figs. 670, 611. 



OPERCULA or AMMONITES. 

 There are constantly found associated with, and generally 

 Lthin the aperture of Ammonites, horny or shelty plates, which 



