54 
The specimens found in England are very different from the 
shell figured by von Buell—to which reference lias already been 
made in Part II.’—which is more like a young specimen 
of Acanthothyris sptinosa, being more coarsely ribbed and 
having fewer spines. The difference can be well seen by 
comparing his figure with those of Davidson, under a magni¬ 
fying lens. They also differ from the varieties of T. senticosa 
of Quenstedt, and from the specimens found in the Oxfoidien 
of France. So we have given the shell, found in the Inferior 
Oolite of England, the specific name, panacanthina. In shape 
it is very constant, and is a most easily recognised shell. Its 
fine, very numerous ribs and spines, its breadth, its small 
somewhat upraised beak, and its very inconspicuous mesial fold 
are most persistent characters which at once distinguish it 
from its ally, Acanthothyris spinosa. A specimen of Acan¬ 
thothyris panacanthina had 100 ribs on the lesser valve, and 
about 40 spines on each rib, thus it possessed at least 4,000 
spines on each valve. 
This species appears to be confined to the district south of 
the Mendip Hills. It is a rare fossil, and it occurs only in the 
Parkinsoni- zone. It is, however, widely distributed, and has 
been obtained in Dorset, at Burton Bradstock, Broadwindsor, 
Clifton Maybank, Bradford Abbas, Halfway House, and 
Combe Quarry, near Sherborne. In Somerset, at Misterton, 
Haselbury, and at Crewkerne Station. 
There are two scarce little species which have been found in 
the Humphresianum- zone in the Sherborne district, namely, 
Phynchonella dundriensis, S. Buckman, and Rhynchonella 
dorsetensis , S. Buckman, which are extremely like Acan¬ 
thothyris senticosa in the general appearance of the test. S. 
S. Buckman, however, has carefully examined specimens 
under the microscope, and states that he has been able to 
detect nothing more than (in one or two specimens of 
Rhynchonella dorsetensis ) what appear to be very minute 
pimples on some of the ribs, and these only very occasionally. 
Still the position of these two species, with regard to the 
other groups of Jurassic Rhynchonella >, is a point by no 
means clear, and the resemblance of their fine-striated test to 
that of Acanthothyris panacanthina is certainly curious. 
