64 JURASSIC RHYNCHONELLIDS 
Internal characters. Pedicle valve. The dental lamellae are apparently only 
developed in the larger species and, when they are not developed, the teeth tend to be 
inserted at a very shallow angle. 
Brachial valve. A low median septum may be present but septalial plates and 
a septalium are never developed. The ends of the inner hinge plates show a dis- 
tinctive thickening and in one species, A. (Acanthorhynchia) spinulosa, this results in 
a structure comparable with a cardinal process. The crura, especially in A. (Acan- 
thorhynchia) panacanthina, the only really well preserved species examined, appear 
to closely resemble the form described by Muir-Wood (1935) as calcarifer. How- 
ever, in other species they seem to more closely resemble the radulifer type. 
Species. The following nominal species are provisionally recognized as belonging 
to the genus ; a sub-generic designation is given where possible : 
. davidsomi (Rollier) (1917, p. 77) 
. (Echinirhynchia) dealbata (Rollier) (1917, p. 80) 
. (Echinirhynchia) fileyensis (Buckman & Walker) (1889, p. 56) 
. (Echinirhynchia) impressata (Rollier) (1917, p. 79) 
. (Echinirhynchia) loriolt (Roller) (1917, p. 79) 
. (Acanthorhynchia) multistriata (Kitchin) (1900, pp. 75-77) pl. xiv, figs. Io, II 
. (Acanthorhynchia) myniacantha (Deslongchamps) (1859, p. 251, pl, iv, fig. 12) 
. nikitensis Gerasimov (1955, p. 216, pl. xliii, fig. 5) 
(Acanthorhynchia) panacanthina (Buckman & Walker) (1889, p. 53) 
(Acanthorhynchia) regans (Szajnocha) (1879, p. 222, pl. vi, figs. I, 2) 
. (Echinirhynchia) senticosa (Schlotheim) (1820, p. 268) 
. (Echinirhynchia) silicea (Quenstedt) (1871, p. 115, pl. xxxix, fig. 66) 
. (Acanthorhynchia) spinulosa (Oppel) (1858, p. 608) 
. (Acanthorhynchia) vilsensis sp. nov. 
. zietent (Rollier) (1917, p. 75). 
eae 
See RRR RRR 
DiIsTRIBUTION. Although rare, the genus is very widespread and occurs through- 
out the French and German Jura, and in Normandy, Poland and England. It has 
also been recorded from the Kutch. 
REMARKS AND OCCURRENCE. The genus was founded by Buckman in 1918, after 
being mentioned by him in 1914. In his original description, Buckman included 
most of the species here retained in the genus, together with a few now considered as 
belonging to Acanthothiris or to be of uncertain affinity. Although distinctive, all 
the included species are rather rare and this has necessitated the use of much 
borrowed material in order to attempt a complete revision of the genus. 
The genus Acanthorhynchia, as here defined, includes two distinct groups, based 
on external morphology, which overlap each other in time and, to a lesser extent, 
geographical area. It is here proposed to designate these groups as sub-genera, 
namely Acanthorhyncia s.s. and Echinirhyncha sub-gen. nov. 
As stated above, all the species are rare and details of the source rock and supposed 
environment are only rarely given in the literature. However, the fact that A. 
(Acanthorhynchia) panacanthina comes from the Inferior Oolite and A. (Acanthor- 
hynchia) rogans comes from the Balin Oolite suggests that the sub-genus Acanthor- 
