Hyatt.] 20 | [January 17, 
individuals attain a diameter of one third of an inch before the sides 
become parallel. 
© 
DACTYLIOCERAS. 
Dactylioceras commune. 
Amm. communis Sow., Min. Conch., vol. v, p. 9, pl. 107, fig. 23. 
Amm. annularis Zeit. Verst. Wurt., p. 14, pl. 10, fig. 10. 
The adults of this species have a narrow abdomen and bifurcated 
pile similar to those of Celoceras mucronatum. ‘The pile are much 
closer set, and not tuberculated, and the sides and abdomen rounder. 
These same differences are very distinctly marked at a very early 
stage. ‘The young are smooth, and resemble the adult of pettos. The 
lateral pile are single, and though tubercles are present in the young 
during the pettos stage, these disappear immediately. It is doubtful 
whether any of the adult pile are ever truly tuberculated. The 
young whorl is very broad in proportion to depth abdomino-dorsally, 
otherwise it closely resembles its own adult. ‘The sides are not di- 
vergent but rounded, and the abdomen rising slightly. The increase . 
is very slow to the adult, whose whorls are rounded and cylindrical. 
Dactylioceras Holandrei. ts 
Amm. Holandrei D’Orb., Terr. Jurass., Ceph., p. 330, pl. 105. 
Dactylioceras Holandrei Hyatt, Bull. Mus. Comp. Zoology, no. v, 
p- 95. 
_ This species is so closely allied to Dactylioceras commune that I can 
see no grounds for their separation, except that both D’Orbigny and 
Oppel have considered them as distinct. The history of the devel- 
opment and all other characteristics are the same, except the flatten- 
ing of the sides of the adult in Dactylioceras Holandrei. 
Dactylioceras annulatum. 
Amm. annulatus Sow., Min. Conch., vol. m1, p. 41, pl. 2238. 
Amm. annulatus D’Orb., Terr. Jurass., Ceph., p. 265, pl. 76. 
Dactylioceras annulatum Hyatt, Bull. Mus. Comp. Zoology, no. v, 
p- 95. 
In some specimens of this species the young appear to have been 
smooth and tuberculated, as in Dactylioceras Holandrei, but in others 
the reverse occurs. The young resemble more decidedly, in the 
single instance examined by me, the adult of Celoceras Desplacei 
than anything else. The lateral pile were bifurcated and coarsely 
tuberculated, as in that species. It resembles Desplacei in the in 
