1872.] 15 | (Hyatt. 
tubercles along the edge of the abdomen, are repeated in the young, 
or during life in all the species which I have examined. There ap- 
pear to be several series, but owing to the manner in which they 
have been found, to a great decree mixed or confused in the same 
formation, it is not possible to pick out the series so clearly. The 
evidence afforded by the geological succession of the species is want- 
ing, and the observation made upon the zoological affinities cannot 
therefore be verified and corrected. : 
The first series may be considered somewhat doubtful. 
Celoceras pettos occurs in the Jamesonibed of the Middle Lias, and 
in the next bed, or Ibexbed, is found the only representative of the 
first series, Celoceras centaurus. ‘This species is referred with much 
doubt to the same genus, on account of certain resemblances of the 
young. Ifthis position is the natural one for the species, it exhibits a 
degraded condition of Celoceras pettos. ‘The second series consists of 
Celoceras Desplacei, and the specimens described under the head of 
Celoceras a and Celoceras 6, from the Middle and Upper Lias. 
This series is closely connected in some of its characteristics with 
Peronoceras, such as the flat abdomen, and divided and tuberculated 
lateral pile of the young. On the other hand the untuberculated 
rotundity and single continuous pile of some varieties of Celoceras 
Despiacei and Celoceras a and 0b in the adults, render it somewhat 
similar to Dactylioceras. The next series, beginning with Celoce- 
ras pettos, and having in it Celoceras mucronatum and crassum, has 
septa with a narrow abdominal lobe; the cells and lobes are remark- 
ably simple, generally bifid, and rather small. Celoceras crassum 
has the young like Celoceras pettos, but the sides become gibbous, 
and in some varieties finally flattened and parallel. In the next 
species, Celoceras mucronatum, these stages are repeated in the 
young, and as the shell nears the adult period the sides become not 
only flattened and parallel, but convergent. 
The next series consist of Dactylioceras commune, Holandret, annu- 
latum and Braunianum, all of the Upper Lias, Posidonomyenbed. 
In the young of Dactylioceras commune we find a smooth, slowly 
increasing whorl, which resembles the young of Peronoceras subarma- 
tum at the same period. A distinct resemblance to Celoceras pettos 
may be shown in the young, and was traceable in well preserved 
specimens, but in others the tubercles were not apparent. ‘The prin- 
cipal features are closely representative of the adult of Caloceras 
mucronatum, especially of the narrow abdomen and tuberculated, but 
