MODERN CLASSIFICATION. 
235 
Genus Thysanoceras, Hyatt. — Abdomen rounded ; whorls exposed ; amount of 
involution about one third of each interior whorl. Ex. Lytoceras fimbriatmn, Sow. 
(PI. XLIV). 
Genus Rhacoceras, Agassiz. — Abdomen rounded ; sides of the whorls flattened ; 
envelopment extends about two thirds over each of the interior whorls, or entirely 
encloses them, covering up the umbilicus. Ex. Phylloceras Loscombi, Sow. (PI. XL, 
fig. 4), Aeyoceras Ibex, Quenst. (PL XXXIX, fig. 4). 
DACTYLOIDiE. 
This family includes the Planulati and part of the Macrocephali. 
Genus Cceloceras, Hyatt. — Pilae on abdomen bifurcated ; lateral pilse single, or 
bifurcated with one external row of tubercles on each ; the young flatter than the adult ; 
the first two whorls smooth, the others tuberculated. Ex. Aegoceras pettos, Quenst. 
(PL XXXVII, figs. 5—7). 
Phtmatoid^. 
Genus Phymatoceras, Hyatt.' — Abdomen rounded or fiattened, never acute; no 
channels in the adult. Young smooth on the first or second whorl, tubercles appear on 
the second or third, and, gradually dividing, spread themselves out upon the abdomen as 
bifurcated pilse, which disappear on the borders of the channels. Keel makes its appear- 
ance at an early age. Ex. Phymatoceras robustum, Hyatt. 
Genus Hammatoceras, Hyatt. — Abdomen rounded or acute, always keeled, but 
never sulcated. Pilse prominent and straight. Envelopment extending over one half the 
width of the sides, or covering only the abdomen of each internal whorl. The two 
species cited are H. insigne, SchubL, and H. variabile, d'Orb. ; these, however, are found 
only in the Jurense-zone of the Upper Lias, and not in the Lower Lias, as our author has 
placed them. 
AMALTHEOIDiE. 
Genus Pleuroceras, Hyatt. — Abdomen flat, with keel and channels well-defined ; 
keel crenulated ; channels vary from obsolete to deep and well-defined. Pilse SAvelling 
below, tuberculated ; genicular bend prominent. Tubercles lateral, arranged along 
the line of envelopment. Umbilicus open. Ex. Amaltheus spinatus, Brug. (PL L), 
the other species cited by our author P. HawsJcerense, costatum, &c., are mere varieties 
that well-known species of Bruguiere. 
Genus Amaltheus, Montfort. — Abdomen acute, keeled, and channelled; whorls 
