262 THE LIAS AMMONITES. 



its origin ; but to indicate the species which forms the connecting hnk between the two 

 genera is absolutely impossible. The near affinity between Peltoceras and PerispMnctes 

 is not only shown by the general appearance of the young specimens of species of the 

 former genus, but also by the occurrence of the faint contractions of the shell in one or 

 other of the Peltocerata. There are, however, yet other genera which take their rise from 

 Perisphinctes, as Aspidoceras and Simoceras, and which are very nearly allied to our 

 genus ; in fact, so much so that I formerly thought it necessary to unite a part of the 

 forms generally comprised under the name of Aspidoceras, the group of Asp. perarmatum 

 and allied species, with Peltoceras. Thus there can be no doubt from the characters 

 exhibited by the younger stages of growth of species of both genera, Aspidoceras and 

 Peltoceras, that the one originates from the group of Perisphinctes Martinsii, d'Orb., to 

 which the larger number of Middle Jurassic PerispJdnctes belong ; the other, on the 

 contrary, takes its origin from a group of Perisphinctes as yet apparently entirely unknown 

 in the European or any other Jurassic strata, and perfectly distinct from the Perisph. 

 Martinsii or Convolutus gvoaTp." Dr. Waagen subdivides the genus into three sections :■ — 

 a. Group of Peltoceras annulare. Rein. h. Group of Peltoceras JEugenii, Raspail. 

 c. Group oi Peltoceras atldeta, Phill. Of the five species comprised in the three sections 

 no less than two, Peltoceras Ardnennense, d'Orb., and Peltoc. atlileta, Phill., are identical 

 with the old European type-forms of these well-known species in our Oxfordian strata. 



Group Simoceras, Zitt. — Shell flat, discoidal, with a wide umbilicus ; ventral side 

 rounded or channelled. Sculpture seldom absent, and consisting for the most part of 

 straight, simple, or divergent ribs, which, in general, during the entire life of the species, 

 or, at any rate, in youth, are unbroken on the ventral side, and abundantly provided with 

 tubercles. They have the last whorl much swollen out; separate from the forward 

 directed contractions observed on all the inner whorls. Body-chamber three-fourths of 

 a whorl in length. Mouth-border in the geologically youngest forms has upward 

 bent external auricles ; in the older forms it is not known. Lobe-line not very 

 complex ; siphonal lobe the largest ; external saddle very much developed and broad. 

 Lateral lobe single-pointed and very small. This genus is found only in the upper 

 division of the Jurassic rocks, commencing with the upper part of the Middle Jura and 

 extending into the Lower Tithonian or Uppermost Jura. The following are the type 

 forms : 



Simoceras anceps, iJem. 



— arthriticum, Sow. 



— admirandum, Zitt. 



— biruncinatum, Quenst. 



— Catrianutn, Zitt. 



— contortum, Neum. 



— explanatum, Neum. 



Simoceras Fraaai, 0pp. 



— Greppini, 0pp. 



— Jooriense, Waag. 



— lytogyrum, Zitt. 



— Rehmanni, 0pp. 



— sulcatum, Hehl. 



— Venetianum, Zitt. 



