56 JURASSIC RHYNCHONELLIDS 
exact horizon is not given but it is probably from the Upper Trigonia Grit, i.e. the 
parkinsoni zone of the Bajocian. 
STRATIGRAPHICAL RANGE. Upper Bajocian to Upper Bathonian/Lower Callovian. 
DIMENSIONS OF NEOTYPE. Length 2-14 cm, thickness 1-71 cm, width 2-37 cm. 
MATERIAL. 30 specimens from the collection of the B.M.(N.H.) ; 23 specimens 
from Normandy and g from Switzerland in the collection of the author in the British 
Museum (Nat. Hist.) nos. BB. 4568289. 
DESCRIPTION. External characters. Medium to large sized Acanthothiris which 
is characteristically fairly globose. In early growth stages the two valves are of 
almost equal convexity but, in adult forms, the brachial valve is much the more 
inflated. Young forms have a subcircular to subtrigonal outline, which becomes 
modified to the subpentagonal shape of the adult. The width is usually, but not 
invariably, greater than the length. There is a marked, rounded uniplication in the 
anterior commissure, accompanied by a low fold on the brachial valve. 
In early growth stages, the beak tends to be small, acute and erect ; it becomes 
more incurved in the adult and often obscures the pedicle opening and deltidial 
plates. The deltidial plates, as seen in the serial sections, are conjunct ; the pedicle 
opening is submesothyridid. Beak ridges are quite well developed and limit the 
incurved interarea. 
The test ornament consists of about 30 to 35 subangular to subrounded ribs, as 
counted at the anterior commissure ; these ribs may bifurcate once or twice, rarely 
more, and occasional ribs are inserted by intercalation. The long, slender spines 
which form the most distinctive feature of the genus arise from expanded bases 
situated along the crests of the ribs. Spines of up to I cm have been observed still 
attached to the test ; however, in most specimens they are indicated only by the 
presence of low spine bases. Although they are hollow, they do not seem to have had 
any direct connection with the mantle. The other ornament consists of rather weak 
growth lines, which are usually only seen in the inter-rib troughs of well preserved 
material, and occasional growth lamellae. 
Davidson (1858) suggested that, “‘ when alive, R. spinosa, was more or less coloured 
with red ; at least, we have seen specimens in which the spines had preserved that 
colour ; this is also the opinion of M. Deslongchamps’’. This observation was 
probably founded largely on material from quarries near Caen in Normandy and, 
while true, omits to point out that large numbers of other organisms are also so 
coloured and this would appear to be due to some local factor rather than to original 
colouration. Dr. Rioult, (personal communication, 1964) suggested that this 
colouration is due to algae. 
DIMENSIONS OF FIGURED ADULT SPECIMENS. 
length thickness width 
2:28 cm 1-78 cm 2°37. cm 
2°14 cm I-7I cm 2°37 cm 
2°42 cm 2°05 cm 2°88 cm 
Internal characters. Pedicle valve. The development of the deltidial plates, 
showing the crumpling together characteristic of the species, is clearly illustrated in 
