96 JURASSIC RHYNCHONELLIDS 
DESCRIPTION. External characters. The most characteristic feature is the strong 
asymmetry exhibited by all the species, as viewed from the anterior. The “ twist ”’ 
occurs indiscriminately either to the right or to the left. A fold and uniplication are 
never developed. Each valve has 20-30 angular or subangular ribs which are 
almost invariably simple, arising from the umbones ; however, bifurcation has 
occasionally been observed postero-laterally in T. guebhardi and T. speciosa. 
The strong beak is usually suberect although in specimens with a very inflated 
brachial valve, notably in T. inconstans, it may be adpressed to that valve. Except 
in the latter case, a functional pedicle opening is present and is flanked by disjunct to 
strongly conjunct deltidial plates. 
Internal characters. Pedicle valve. The lateral cavities and dental lamellae are 
rather variably developed, ranging between massive lamellae and correspondingly 
small lateral cavities and larger cavities bounded by slender lamellae. A slight 
pedicle collar may be developed. Strong, crenulated teeth and well developed 
lateral denticulae characterize the hinge apparatus. 
Brachial valve. A low, relatively long median septum is present ; septalial plates 
occur in all the species investigated, although they are of a distinctively reduced 
form. Inner and outer socket ridges are both well developed ; the socket floors 
are strongly crenulated. 
The crural bases differ quite markedly in their development in the different 
species described. Although the crura are radulifer in general appearance, they 
show considerable variation towards their distal ends. It is not known whether 
the development of these various distal processes can be regarded as consistent at 
species level. 
Species. The following nominal species are attributed to the genus : 
T. “ astieriana”’ (Orbigny) (in Haas, 1891, p. 62, pl. vii, figs. 3-6, pl. ix, figs. 
1-9) 
T. “ astieriana’’ (Orbigny) (in Makridin, 1964, pp. 107-9, pl. ui, fig. 8) 
T. “ astieriana”’ (Orbigny) (in Wisniewska, 1932, p. 20, pl. i, figs. 21-26) 
astiertformis (Wisniewska) (1932, pp. 27-28, pl. i, figs. 19 ?20) 
inconstans (Sowerby) (1821, vol. III, p. 137, pl. 277, fig. 4) 
guebhardi (Jacob & Fallot) (1913, p. 44, pl. v, fig. 9) 
. pseudo-inconstans (Kitchin) (1900, pp. 51-2, pl. x, figs. 6-10) 
. lehmanni (Makridin) (1964, pp. 109-10, pl. ii, fig. 9) 
. pectunculoides (Etallon) (1861, p. 289, pl. xlii, fig. 3) 
. semiconstans (Etallon) (1861, p. 290, pl. xlii, fig. 4) 
. speciosa (Minster) (1839, p. 113, pl. xiii, fig. 6). 
DistTRIBUTION. Although individual species appear to be relatively restricted 
geographically, the genus, taken as a whole, occurred over much of Europe west of 
the Alps. If the species of Makridin mentioned above have been correctly assigned, 
then the genus was also present on the Russian Platform. 
OccURRENCE. The genus appears, from the limited information available, to have 
been restricted to peri-reefal areas, and it is suggested that the asymmetrical develop- 
ment may represent an adaptation for living in a high energy environment. 
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