1872.] 13} (Hyatt, 
SERIES PERONOCERINZ. 
This series has a development which connects it genetically with 
the Liparoceratidee, but the adults possess the characteristic pile and 
general aspect of the family of Dactyloide. They combine the 
characteristics of both the Dactyloide and Liparoceratide without, 
however, being in any sense, so far as I can judge, transition types. 
The adults have the Dactyloidan characteristics, but these appear to 
be only mimetic. All the species of this last family, which are the 
real ancestors of the Perisphinctes group through Celoceras Pettos, 
never repeat any of the characteristic features of Peronoceras in the 
course of their development, nor, on the other hand, do the young 
of Peronoceras show any genetic connection with vither the young 
or the adults of the Dactyloide. 
This series consists of Peronoceras acanthopsis and subarmatum, 
all of the Upper Lias, and found in the Posedonomyenbed. The 
horizon of acanthopsis is doubtful, but as this species is generally 
considered identical, and may really be a variety of subarmatum, it is 
probable that it is associated with the latter in the Posedonomyen- 
bed. The adult of acanthopsis is similar, as shown in the specific 
description, to the young of subarmatum, which resembles it in the 
flatness and breadth of the abdomen, divergency of the sides, etc. 
The young of Peronoceras subarmatum has, before the period at 
which it resembles acanthopsis, smooth, cylindrical whorls, then a 
period in which the highly divergent, tuberculated, but partly 
smooth sides remind one of Deroceras muticum. ‘The slow increase 
of the whorls during the smooth period reminds the observer at 
once of the young Deroceras armatum, and the subsequent resem- 
blance to the adult of Deroceras muticum, together with the near 
affinity of the septal outlines, appears to settle the question of deri- 
vation. 
PERONOCERAS. 
Peronoceras acanthopsis. 
Amm. acanthopsis \YOrb., Prod. d. Pal. Stratigraphique, p. 247. 
Deroceras acanthopsis Hyatt, Bull. Mus. Comp. Zoology, no. v, p. 94. 
One specimen of this species agreeing precisely with D’Orbigny’s 
description, was found in Bronn’s collection, labelled “ subarmatus.” 
Ii differs from this species, however, exactly as D’Orbigny de- 
