7 
he could have very greatly increased the usefulness of his lists; because 
generic names, as he shows by those that he does employ, tend to narrow 
the field of bibliographic research. 
The original source of the Macroeephalitidae may be looked for in 
the Bajocian coronates: the forms of the family have presumably passed 
through stages of development similar to those shown in the Sphaero- 
ceratidae, particularly in the genus Labyrinthoceras S. Buckman (PI. 
CXXXIV), but the Macroeephalitidae are at present only known in the 
costate state, congruous with L. perexpansum S. Buckman (op. cit.) and 
are not known in the coronate stage, exhibited by L . extensum S. Buckman 
(PI. CCXIV). 
On this theory, then, the Macroeephalitidae are, as regards their 
ribbing, in the post-coronate, or post-tuberculate stage and no forms in 
the coronate stage are known, though the young example figured by Blake 
(Macrocepkalites macrocephalus, Pl. III, 4) comes very near to the cadicone 
coronate which this theory postulates. 
In regard to suture-line the Macroeephalitidae show a character 
pertaining to the Sphaeroceratidae, the Tulitidae, and the Pachyceratidae — 
feeble development of the backward curvature of the umbilical lobes: in 
the Btepheoceratidae this character is very strongly developed. 
In geological position the Macroeephalitidae follow the Tulitidae; 
but are separated from them by part of the Great Oolite, by the Forest 
Marble, and by the main of the Cornbrash— formations which represent a 
far longer interval of time than has hitherto been supposed. That the 
Tulitidae can produce macrocephalitoid forms is shown by the. genera 
Pleurophorites, Morrisites, and Morrisiceras S. Buckman (3, pp. 43-48); 
but the feebleness of ribbing in the Tulitidae is against that family being 
regarded as the direct ancestor of Macroeephalitidae. A form like Tulites 
cadus B. Buckman (p. 45) with ribs much stronger and running straight, 
not curved, across the venter, may be expected as the ancestral form of 
the Macroeephalitidae. 
Evidence for straight ventral ribs as the primary feature in Macro- 
eephalitidae is shown by young examples figured by Blake (PL III, 2, 14, 
3, IV, 5), and may be inferred in others of which he does not give ventral 
views (PI. Ill, 4, PL III, 5). In fact, none of his Macrocepkalites , large or 
small, seems to show anything but ribs straight across venter. 
To Cadoceratidae the Macroeephalitidae have a superficial resemb- 
lance at times, but the manner of development is different: the Cadocera- 
tidae pass from platycone with sometimes sharp almost carinate venter to 
cadicone or sphaerocone, losing ribbing with increase of thickness; the 
Macroeephalitidae pass from sphaerocone to platycone, losing ribs as 
thickness declines. 
From certain Kosmoceratidae with macrocephalitoid aspect the 
Macroeephalitidae differ by showing, in the inner whorls, no sign of the 
flattened zone on the venter, which is known as the runcinate stage. More- 
over, the Kosmoceratidae generally show decided lateral uni- or bi-tubercu- 
lation and a simple suture-line. One, sometimes two, of these characters 
will be present to reveal a Kosmoceratid in cases of doubt, as when the 
runcinate venter of inner whorls is not visible. 
