98 JURASSIC RHYNCHONELLIDS 
The asymmetrical development usually commences when the animal is about half 
grown and becomes increasingly pronounced with age. 
DIMENSIONS OF FIGURED SPECIMENS. 
length thickness width 
3°20 cm 3°10 cm 3°25 cm 
3°02 cm 2°64 cm 3°12 cm 
3°20 cm 2:25 cm 3°31 cm 
Internal characters. Pedicle valve. The shell wall is very thick and the small 
lateral cavities are not seen in transverse section until about the level of the top of 
the brachial valve. The dental lamellae are massive and only attached to the 
ventral shell wall at their extreme posterior ends. The teeth are strong and heavily 
crenulated ; the general impression given by the teeth, dental lamellae and the way 
the pedicle valve abuts against the brachial valve in the earlier sections, is that the 
species possessed a very strongly articulated shell. 
Brachial valve. The sockets are crenulated and there are both inner and outer 
socket ridges. A median septum is present but it does not come into contact with the 
septalial plates as it is only present posteriorly as a low ridge. The septum com- 
pletely disappears, as seen in transverse section, before the end of the crura is 
reached, but shows quite a strong development for part ofits length. The distinctive 
short, rounded septalial plates are seen on the dorsal side of the ventrally curved 
hinge plates. Crural bases are hardly differentiated. At first, as seen in transverse 
section, the radulifer crura appear as structures flattened in the plane of articulation, 
but in later sections they become more rod-like and finally become flattened at right 
angles to the plane of articulation and curve towards the pedicle valve. 
DISTRIBUTION. The best known localities are those of Ringstead Bay and 
Weymouth in Dorset ; inland, it has been recorded from Shotover Hill near Oxford, 
Wooton Basset in Wiltshire, Brill in Buckinghamshire and from Swindon. On the 
continent as observed by Haas (1890), the species appears to be restricted to north- 
western France. Buch (1838) recorded it from Ellrichserbring in Brunswick. His 
figure of the species is a copy of Sowerby’s (1821) figure 4 and, as he did not illustrate 
any material from his locality, the record must be regarded as doubtful. 
OccuURRENCE. At Ringstead Bay it occurs in a thin bed of pale grey clay, lying 
between the Ringstead Coral Bed and the Exogyra Bed, as illustrated in Arkell 
(1933, pl. 21). The associated fauna includes terebratulids, the gasteropod Bathroto- 
maria reticulata (Sowerby) and ammonites. Ager (1965a) has drawn attention to 
the probability of 7. inconstans living in a peri-reefal environment, as evidenced by 
the close proximity of the Coral Bed. At Shotover Hill it is recorded by Phillips 
(1855) as occurring in a band of septarian nodules in a shaly clay of cymodoce age. 
RemaRKs. In his discussion of the species, Davidson (1852) included a specimen 
(pl. 18, fig. 4) from the Inferior Oolite of Leckhampton Hill. The author has not 
seen the actual specimen, but assumes this to be a misidentification in view of the 
otherwise restricted record of the species. On the same plate fig. 3, Davidson 
figured a symmetrical specimen from Shotover Hill, again the author has not seen the 
