74 JURASSIC RHYNCHONELLIDS 
TYPE SPECIMEN. Neotype (pl. 7, fig. 1), here designated, is the specimen figured 
by Rothpletz (1886) pl. 15, fig. 5, which is No. AS vii 330 in the collection of the 
Bayerische Staatsammlung fiir Palaontologie und historische Geologie in Munich. 
It is considered desirable to designate a neotype as the name senticosa has been used 
very loosely in the past for widely differing forms. 
MATERIAL. 12 specimens from the collection of the University of Erlangen ; 11 
specimens from the Bayerische Staatsammlung, Munich. 
DescripTion. External characters. The following is a translation of the original 
description by Schlotheim, “ Partly in very complete examples, with perfectly 
preserved shells, from Grumbach, near Amberg, in the Pfalz, petrified in chert and 
probably belonging to the Jura formation. This very rare and remarkable Tere- 
bratulite, only recently discovered, has a somewhat elliptical, almost pear-shaped 
form, it is not particularly thick, is regularly convex with a fine, evenly striated, 
shagreen like surface and on both valves the somewhat prominent rays are covered 
with numerous, small sharp spines, by which this and spinosus may be distinguished 
from all other known species. It is very seldom found ”’. 
The Schlotheim description is quite accurate and mentions the two most distinctive 
characters, namely the pyriform shape and the fine, spine-covered ribs. The ribs 
characteristically bifurcate laterally thereby maintaining an even distribution over 
the surface. The species is also characterized by the possession of a rectimarginate 
anterior commisure. The beak is suberect and does not obscure the triangular 
delthyrial opening ; no deltidial plates have been observed. Beak ridges and inter- 
area are not developed. Some specimens have a few growth lamellae developed, but 
growth lines are not present ; it is possible that the latter may have been obliterated 
as a result of the silicification suffered by all the specimens examined. The valves 
are about equally biconvex, although the brachial valve tends to be rather flat and 
may be the less inflated. 
DIMENSIONS OF FIGURED SPECIMENS. 
length thickness width 
1°35 cm 0-61 cm i-r7 Em 
I-94 cm 0-87 cm 1-62 cm 
1°35 cm 0:67 cm I-22, cin 
The last set of figures are the dimensions of the neotype. 
Internal characters. The specimen sectioned was from Heiligenstadt, Franconia, 
and had a silicified shell and dolomitised infilling. This preservation has resulted in 
the loss of all fine detail and it has only been possible to give rather generalized 
drawings of the internal structures. 
Pedicle valve. There are no dental lamellae and the large teeth are inserted 
laterally at a shallow angle. 
Brachial valve. A cardinal process is present. Inner and outer socket ridges 
are both well developed. The form of the crura could not be determined as they 
disappeared into the recrystallized matrix immediately below the last section shown 
in text-fig. 26—top. 
DISTRIBUTION. The Franconian and Swabian Jura. 
