54 JURASSIC RHYNCHONELLIDS 
details of the pedicle opening and deltidial plates. Beak ridges and interarea are 
present but rather variable in development. The young forms tend to be sub- 
trigonal to subcircular. 
Internal characters. Pedicle valve. The delthyrial cavity is quadrate in trans- 
verse section, but in the initial posterior sections there is a low dorsally directed 
ridge. This results from the presence of a groove which is situated on the antero- 
dorsal side of the beak and runs from the pedicle opening to the tip of the beak. 
This structure appears to be a pedicle collar as originally defined by Thomson (1927) ; 
it is not, however, the structure defined as a pedicle collar in the “ Treatise ’’ (1965), 
and is here referred to as the “ pedicle trough ”’. 
The lateral cavities are relatively narrow and the slender dental lamellae bounding 
them are either subparallel or slightly divergent ventrally. Teeth and sockets are 
both crenulated. 
Brachial valve. A median septum is quite well developed. A septalium is not 
present but the median structure, dorsal of the ventrally arched hinge plates, seen in 
A. spinosa and A. cf. A. costata could possibly have resulted from the fusion of 
septalial plates. Crural bases are hardly differentiated. The crura are of the 
radulifer type and tend to be oval or flattened in the plane of articulation, as seen in 
transverse section. Muscle scars have not been observed and even Buckman (1918) 
commented that they were obscure in both valves. 
SPECIES. As wellas A. spinosa and A. cf. A. costata, here described, the literature 
abounds with descriptions and names of spinose rhynchonellids of this age. The 
following nominal species are provisionally attributed to this genus : 
. balinensis Rollier (1917, p. 77) 
. broughensis Muir-Wood (1952, pl. 123, p. v, figs. II, 13-15) 
. crosst (Walker) (1869, p. 215) 
. costata Orbigny (1850, p. 286) 
. doultingensis (Richardson and Walker) (1907, p. 426, pl. xxvu, fig. 2) 
. elargata Seifert (1963, p. 170, pl. x, fig. 8) 
. globosa Buckman (1918, p. 918, pl. xix, fig. 25) 
inflata (Quenstedt) (1868, p. 112, pl. xxxix, figs. 46-47) 
midfordensis (Richardson & Walker) (1907, p. 427, pl. xxvii, fig. I) 
multispinosa Seifert (1963, p. 172, pl. x, fig. 12) 
. oligacantha (Branco) (1879, p. 127, pl. vi, figs. 6-7) 
. paucispina Buckman and Walker (1889, pp. 52-53) 
. powerstockensis Buckman and Walker (1889, p. 52) 
vadwanowticensts Rollier (1917, p. 77) 
. sentosa (Quenstedt) (1868, p. 113, pl. xxxix, figs. 55-56) 
. sinuata (Quenstedt) (1868, p. 114, pl. xxxix, fig. 59) 
. subglobosa Seifert (1963, p. 171, pl. x, fig. 11) 
. tenuispina (Waagen) 1867, p. 640, pl. xxxii, fig. 6). 
DISTRIBUTION. Acanthothiris is a common genus occurring widely throughout 
the Middle Jurassic rocks of Europe, including those of England, Normandy, the 
French, Swiss and German Jura and Poland. 
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