Hyatt.] 14 [January 17, 
scribed. The abdomen is much broader, and the pile are much more 
numerous and closely crowded. They are gathered into knots at 
the spines from both the abdomen and the sides. At irregular in- 
tervals in the young a single pila will cross the abdomen between 
the spines, aud this seems to become constant in the full grown shell. 
Whether it is to be considered a lower form of the series than Pero- 
noceras subarmatum cannot be definitely decided. The adult is 
similar to the young of that species, but the young in-our single 
specimen is wanting, and it cannot be decided therefore whether it is 
an arrested development of Peronoceras subarmatum, or simply an 
undeveloped lower form. The precise geological horizon is uncer- 
tain, Oppel not mentioning the name at all. 
Peronoceras subarmatum. 
Amm. subarmatus Young & Bird, Geol. Yorkshire, p. 250, pl. 13, 
fig. 3. 
Amm. subarmatus Sow, Min. Conch., vol. rv, p. 146, pl. 407. 
Amm. fibulatus Sow., Min. Conch., vol. rv, p. 147, pl. 407, figs. 3, 4. 
Peronoceras subarmatum Hyatt, Bull. Mus. Comp. Zoology, no. v, 
p- 89. 
The separation of the two forms fibulatus and subarmatus, seems to 
me unnatural, though Oppel evidently considered them entirely dis- 
tinct. His material was assuredly better than mine, but nevertheless 
I venture to think that in this case he erred. In the Museum col- 
lection there are nine specimens only, but nearly every specimen is 
distinct from every other, and if “ fibulatus,” or the flattened variety, 
is a distinct species, so are the others. The range also is here much 
less, both in form and characteristics, than between the variations of 
Celoceras Desplacei. The young are either like Peronoceras acan- 
thopsis, or they have exceedingly broad abdomens, divergent sides, 
pile similar, numerous whorls on account of the slow increase of the 
animal in thickness, or dorso-abdominally. The pile, however, are 
single in the young, and the tubercles do not gather them into knots 
though present at an early period. D’Orbieny quotes this species 
from the Middle Lias, but as shown, it occurs only in the Upper Lias. 
DACTYLOIDZ. 
Celoceras pettos appears to be the central form of this family; cer- 
tainly its principal characteristics, the smooth, rotund abdomen, 
divergent and smooth sides, and single line of prominent lateral 
