FROM NORTHWESTERN EUROPE 61 
commissure. These ribs are rather rounded in cross section and frequently bifurcate 
once or twice over their length ; ribs are also occasionally inserted by intercalation. 
The ornament also includes growth lines, but these are not strongly developed and 
are only seen on well preserved material. Long, slender spines arise from the crests 
of the ribs and are fairly evenly distributed over the test ; the hollow nature of these 
is clearly seen in the serial sections. There is a low rounded uniplication in the 
anterior commissure ; however, any accompanying fold in the brachial valve is only 
poorly developed. 
DIMENSIONS OF FIGURED SPECIMENS. 
length thickness width 
1:96 cm 1°45 cm 2°57 cm 
1:87 cm 1:38 cm 2°50 cm 
1-84 cm I'I5 cm 2°30 cm 
Internal characters. Pedicle valve. The deltidial plates, as seen in serial section, 
are very weakly developed, as compared with A. spinosa, and do not meet and 
become crumpled together. The nature of the “ pedicle trough ”’ is clearly seen 
but from the incurvature of the valve it could obviously have been functional only 
during the early stages of ontogeny as proved by the inflation of the brachial valve 
and the incurvature of the beak which must have lead to atrophy of the pedicle in 
adults. The dental lamellae diverge slightly ventrally and limit a subquadrate 
delthyrial cavity. The teeth are strong and crenulated. 
Brachial valve. The median septum is quite strongly developed. There is no 
septalium as the septalial plates, although present, are very weak. Crural bases are 
not differentiated. The crura are radulifer and curve distally toward the pedicle 
valve ; at their proximal ends they are rather flattened in cross section and lie in the 
plane of articulation. 
DISTRIBUTION. The material on which Orbigny based his original description 
came from Gueret (Sarthe) ; all the specimens described by the author came from 
the southern French Jura (Ain). 
OccuRRENCE. Details of the type locality for A. costata are not known. The 
Jura material was collected from the Middle Chanaz Beds (Ager & Evamy, 1963) 
from the area around Belley (Ain). Dr. Evamy gives the following description of 
the lithology in his thesis: “ irregularly bedded, poorly sorted calc-arenites, charac- 
terized... by their content of ferruginous ooliths. ....The lower and principal 
unit ...consists of irregularly bedded brown and grey limestones and similarly 
coloured shales’”’. He also notes, ‘‘ The member (Middle Chanaz Beds) may also 
be recognized in the field by the abundance of Acanthothiris ”’. 
The acanthothirids occurred as clusters or “‘ nests ’’ in the limestones and concen- 
trations of them were also noticed in the shales. In both situations the forms were 
mainly adult with valves intact and, in the majority of cases, with the beaks pointing 
downwards. It would seem that most of the specimens were not far removed from 
their life position ; however, some movement or sorting is indicated by the notice- 
able lack of juvenile forms and the similar lack of in situ spines. It should be noted 
