12 Davidson—Recent and Tertiary Species of Thecidium. 
the anterior fifth. In front ‘and beneath the beak (fig. 4, a) a 
rather strong cervical ridge runs obliquely for a third of an inch; 
and behind the beak two strong ridges, terminating on the 
posterior margin in projecting spines, run down the posterior 
slope. Above these the posterior upper angle is perfectly 
square; beneath them the lower edge is strongly emarginate ; 
the ventral margin arched; and the anterior angles both abrupt 
and square (the upper one even acute), from the deep emargi- 
nation which truncates the whole anterior side. Surface 
marked with concentric ridges, not very closely placed, nor 
very distinct in all parts. 
Though we have but a single valve of this curious fossil, it 
must not be neglected. It is strongly characterized; and in 
every part it seems to have the characters of the genus Ceratio- 
caris exaggerated. The subcardinal ridge is so much stronger 
than in any species of Ceratiocaris I am acquainted with, that 
I am compelled to give a new name. 
I am indebted to R. H. Valpy, Esq., F.G.S., of Ilfracombe, 
for acquaintance with this fossil. He found it some time ago 
in the pebbles, on the banks of the Otter, and suggests the 
trivial name. It is not unlike a Bivalve Shell. Mr. Henry 
Woodward made the drawing from which our woodcut is exe- 
cuted, and I am indebted to him for again calling my attention 
to the specimen. 
Ribeiria pholadiformis, Sharpe, Quart. Journ. Geol. Soc., 
vol. ix. pl. 9, fig. 17, may very possibly be a Crustacean of 
this group, with a remarkably thickened dorsal region and 
strong interior cervical ridge. It has been variously assigned 
to the Anatinide, among Bivalves, and the Calyptreide, 
among Univalves, neither of which groups seems very fit to 
receive it. I do not think we have quite got at the true affinity 
yet; but suggest the above, the more willingly in this place, as 
Ribeiria is a characteristic member of the mid-European Silu- 
rian fauna to which the Normandy fossils and the contents of 
the pebbles at Budleigh Salterton belong. 
On tHE Recent AND TerRTIARY SPECIES OF THE GENUS 
‘THECIDIUM. 
By Tuomas Davinson, F.R.S., F.G.8., &e. 
HE object of the present communication is to review and 
compare the characters of the Tertiary and Recent species 
of Thecidium, the true characters of. which have been discovered 
only within the last few years. The genus was first named 
