1872. | 19 [Hyatt. 
ereases In breadth, so as to. become perceptibly narrower than the 
next inner volution. Two specimens were examined, one from Middle 
Lias, Milhaud Coll. De Konnick, the other from Plateau de Larzac, 
Upper Lias. The septa hardly differ from those of the adult of Ce- 
loceras pettos. A specimen of Celoceras crassum, having the same 
contracted outer whorl, seems to show that these may be merely 
dwarfed varieties of Celoceras Desplacei; the septa also show that 
the diminished end is not a body chamber, but part of a true whorl. 
Coeloceras b. 
This species from Plateau de Larzac appears to be different from 
all other forms. ‘The young for a variable, but rather long period, 
probably five or six whorls, have the prominent tubercles, slight, lat- 
eral and divided abdominal pile of Celoceras pettos. ‘The form, also, 
of the whorls during this time is like that of the adult of pettos. On 
about the sixth or seventh whor! the tubercles disappear, and the pile, 
instead of splitting invariably at the abdominal border, often dichoto- 
mize on the side, as in the advanced age of certain varieties of 
Celoceras Desplacei. -At the same time several may be observed 
intermingled with these, which pass entirely around the whorl without 
division. In the course of a quarter of a volution the transition is 
completed, and all the pile have this character. 
Cceloceras crassuim. 
Amm. crassus Phil. Geol. York., p. 12, fig..15. 
Amm. raquinianus D’Orb., Terr. Jurass., p. 332, pl. 106: 
This species increases the abdomino-dorsal diameter faster, and is 
therefore a larger and stouter shell than Celoceras pettos. The pile 
also.on the sides, though still single, and each one bearing a tuber- 
cele, are more distinctly marked. On the abdomen two or three unite 
at each tubercle, and the intermediate spaces are occupied by single 
pile, which, however, do not extend on to the sides. In the adult 
the sides become parallel. They are at first divergent, then gib- 
bous, and finally flattened and parallel. 
Coeloceras mucronatum. 
Amm. mucronatus D’Orb., Terr. Jurass., Ceph., pl. 104. 
Caloceras mucronatum Hyatt, Bull. Mus. Comp. Zoology, no. v, 
p- 95. 
The young of this species is like the adult of pettos, then as it 
Increases in size-resembles with great exactitude the adult of Celoce- 
ras crassum, and finally the sides become parallel and flat, and begin 
to exhibit their own specially characteristic convergence. Some. 
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