248 MOLLUSCA. CEPHALOPODA. Oubulita. 
96. A. rhotamagensis. — Discoid, radiated, umbilicated ; inner whorls partly 
concealed ; radii about 20, furnished with three short tubercles upon the front, 
and two, more or less elevated, upon each side ; whorls thick, with flattish 
sides ; aperture oblong. Cuv. Oss. Foss. ii. 319. t. vi. f. 2. A. Sussexiensis, 
Mantell. Suss. 114. t. xx. f. 2. t. xxi. f. 10. A. rhot. Sower. Min. Conch, t. dxv. 
— Chalk-marl. 
97. A.Benettiana — Sower. Min. Conch, t. Dxxxix — ^No description as yet 
given. 
98. A. hipUcatus. — Depressed, slightly umbilicate ; volutions inserted, trans- 
versely radiated ; rays prominent, curved, bifurcated, arising from a row of 
oblong projections on the inner edge of the volutions, and terminating in 
tubercles on the outer margin ; carene flat, bordered by alternating, com- 
pressed tubercles. — Mant. Suss. 91. t. xxii. f. 6 — In blue chalk-marl. 
99. A^ Woollgari — Discoidal, depressed, volutions one-third inserted, trans- 
versely costaLed : costae remote, slightly curved, inclined towards the aper- 
ture, terminating on the outer margin, in compressed tubercles, or spinous 
projections ; carene acute, deeply serrated. — Mant. Suss. 197- t. xxi. f. 16. 
t. xxii. f. T.— -Upper chalk. 
100. A. planorbis — Discoid, smooth ; whorls two or four, two-thirds exposed 
Sower. Min. Conch, t. ccccxlviii. — In Lias^ at Walchel. Probably not of the 
genus, or even of the group Cephalopoda. 
It is probable that not a few of the preceding species will, upon more ac- 
curate comparison, be degraded to the rank of varieties. The following ob- 
servation by Mr Sowerby merits attentive consideration. “ There appears 
to be no regular rule amongst Ammonites for their change of form, some be- 
coming more globose, and others more compressed by age ; but they generally 
lose some of the ornaments from their last whorls ; and, in their infant state, 
are also smooth, or free from tubercles.” t. 405, 
Gen. ORBULITA. — Sides equal, the last whorl embracing 
and concealing the previously formed ones. 
1 . O. disca. — Discoid, outer edge acuminated ; aperture sagittate, half . 
the diameter of the shell in length, and one-sixth in breadth. — Ammonitis 
discus, Sower. Min, Conch, t. xii. — In limestone of the Lower Oolite., Bed- 
ford. 
2. O. striata — Discoid, gibbose, obscurely undulated, finely striated lon- 
gitudinally ; septa rather distinct, with four large angular folds. Aperture 
semicircular, with nearly parallel edges ; siphunculus marginal ; a deep cen- 
tral cavity. — Am. str. Sower. Min. Conch, t. liii. f. 1 — In Carboniferous Lime- 
stone., Derbyshire. In the Min. Conch, it is stated, at t. 130, that Dr Buck- 
land has found this species in Transition Slate at Filliagh, near South Mol- 
ton, Devonshire. 
3. O, sphcerica. — -Orbicular, umbilicate ; septa with four broad angular 
folds ; aperture narrow ; syphon on the inner margin Conchyliohthus 
Nautilus sphsericus, Martin., Pet. Derb. t. vii. f. 3, 4, 5. Am. sph. Sower. Min. 
Conch, t. liii. f. 2.— Carboniferous Limestone, Derbyshire. 
4. O. minuta. — Oibicular, with about 24 distant longitudinal striae ; aper- 
ture lunate, rounded at the sides. — Am. min. Sower. Min. Conch, liii. f. 3 
In Chalk Marl, F olkstone. 
5. O. modiolaris — Orbicular, central cavity large, with an angular edge, 
exposing the inner whorls ; septa numerous, Avith five principal undulations, 
