262 
THE LIAS AMMONITES. 
its origin ; but to indicate the species wliicli forms the connecting link between the two 
genera is absolutely impossible. The near affinity between Peltoceras and P erispliinctes 
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 
Perisj)hinctes, 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. perarniatum 
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 PerispJmictes 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 PerisjjJnnctes as yet apparently entirely unknown 
in the European or any other Jurassic strata, and perfectly distinct from the Perisph. 
Martinsii or Convohitus grou^ Dr. Waagen subdivides the genus into three sections : — 
a. Group of Peltoceras annulare, Rein. d. Group of Peltoceras Eugenii, Raspail. 
c. Group of Peltoceras athleta, Phill. Of the five species comprised in the three sections 
no less than two, Peltoceras Arduemmise, d'Orb., and Peltoc. at/ileta, VhiW., are identical 
with the old European type-forms of these well-known species in our Oxforclian 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 aiiceps, Rein. 
— artliriticum, Sow. 
— admirandum, Zitt. 
— biruncinatum, Quenst. 
— Catrianum, Zitt. 
— contortum, Neum. 
— explanatum, Neum. 
Simoceras Fraasi, Ojjp. 
— Greppini, 0pp. 
— Jooriense, Waag. 
— lytogyi'um, Zitt. 
— Rehmauui, 0pp. 
— sulcatum, Hehl. 
— Venetiainim, Zitt. 
