108 JURASSIC RHYNCHONELLIDS 
DIMENSIONS OF HOLOTYPE. Length 2:50 cm, thickness 1-78 cm, width 2:20 cm. 
MATERIAL. 20 specimens collected by the author from the type locality, British 
Museum (Nat. Hist.) nos. BB. 45710-19, BB. 45770-79 ; 30 specimens from near 
Bourges (Cher) collected by M. Delance, (University of Dijon). 
DESCRIPTION. External characters. The species is an ordinary looking rhyn- 
chonellid with the valves either equally biconvex or with the brachial valve slightly 
the more inflated. Length and width are about equal and either may be the greater; 
maximum width may be well toward the anterior or roughly median, thus giving a 
considerable range of overall outline. The young specimens are more flattened and 
subtriangular. 
Most specimens have a high, strong beak which is flanked by strong beak ridges. 
The interarea is markedly incurved and is developed most strongly at the base of the 
beak where it impinges on the brachial valve giving a distinctive bend to the hinge 
line. A large, circular, hypothyridid pedicle opening is present and is limited by 
conjunct deltidial plates. 
The radial ornament consists of rather coarse, steep, simple ribbing, while the 
concentric ornament comprises numerous very fine growth lines. The anterior 
commissure either shows a symmetrical low uniplication or varying degrees of 
asymmetry ; the latter feature is never so strongly developed as in Torquirhynchia, 
but shows the same sort of variation as described for Septaliphoria. 
DIMENSIONS OF FIGURED SPECIMENS. 
length thickness width 
1:26 cm 0-51 cm 1-25 cm 
1-82 cm 0-77 cm 1:66 cm 
2°40 cm 1°85 cm 2°17 cm 
2:28 cm I'42 cm 2:18 cm 
2°50 cm 1-78 cm 2°20 cm 
Internal characters. Pedicle valve. The beak is massive and the sub-parallel 
dental lamellae are quite short resulting in the development of only small lateral 
cavities. A small pedicle collar is present. The teeth are strong and crenulated and 
the species also possesses strong inner and outer socket ridges. 
Brachial valve. The hinge plates are arched ventrally and the inner hinge plate 
is distinctive in showing a dorsally directed ridge flanked by small septalial plates. 
Crural bases are differentiated ; the relatively small radulifer crura curve slightly 
toward the ventral valve. 
DISTRIBUTION. Only known from the type locality and from near Bourges (Cher), 
France. 
OccuRRENCE. The specimens from Isére were collected from a band of fine- 
grained limestone, about 1-25 m thick, in which they occurred as loose groups. The 
associated fauna consisted of occasional zeilleriids and lamellibranchs, including 
oysters. M. Delance, of the University of Dijon, describes his material as coming 
from a “ calcaire crayeux blanc, trés friable ’’, which lies above a massive reef lime- 
stone containing corals. 
