18 Davidson—Recent and Tertiary Species of Thecidium. 
The interior of the dorsal valve is slightly concave, with an ob- 
long, square-shaped, prominent, cardinal process, between the dental 
sockets. A broad, thickened, raised, granulated margin encircles the 
valve, and forms a bridge over the small, deep, visceral cavity, close 
to the basis of the cardinal process; the granulations are larger and 
more prominent on the inner margin. On reaching the front, near 
the middle, the margin suddenly curves upwards on either side, and 
unites so as to form a central a-shaped ascending process, the at- 
tenuated extremity of which is directed towards the middle of the 
bridge-shaped process already described. The descending apparatus 
consists of two oval pectinated ridges (Pl. IL. fig. 2, 2, k), united close 
to the extremity of the central angular ascending process, and fol- 
lowing ata little distance the curves of the inner margin of the same 
ascending process. Proportions variable ; length and breadth about 
two lines. . 
In external shape this Thecidium cannot be distinguished from the 
Mediterranean species; but its interior is very different, and resem- 
bles by its simple arrangements that of several Jurassic forms, such 
as Thecidium Moorei, Th. triangulare, &c. 
Thecidium Barretti was obtained by the late Lucas Barrett at 
60 fathoms on the north-east coast of Jamaica, and was found by 
him fossil in the newest Pliocene beds of the same country. 
TERTIARY SPECIES AND VARIETIES OF THECIDIUM. 
Three so-termed species have been found in the Ter- 
tiary strata; one of these was discovered by Sign. Michelotti 
in the Middle Miocene of the hills near Turin; and this he 
designated Thecidea testudinaria. The same shell was also 
found by Sign. Meneghini in the Miocene beds of Parlascio in 
Tuscany. Another, Thecidium Adamsi, Macdonald, occurs in 
the Miocene beds of Malta; and a third, not yet described, 
was found by Ad. von Koenen, in the Lower Oligocene beds 
(next above the London Clay) at Latdorf, near Bernburg, in 
the North of Germany. 
Having obtained by the kindness of Sign. Michelotti several 
specimens of the Turin shell, also many examples of the Maltese 
Thecidium from Dr. Adams, and a few specimens of that from 
Latdorf from Messrs. von Koenen and Bosquet, I have been 
enabled to study and compare all their external and interior 
details with all possible attention, and of these I now offer 
carefully enlarged illustrations. 
1. THEectipium MrpDITERRANEUM, var. LATDORFIENSE. 
Pl. I., figs. 6-9. 
In external shape, dimensions, and internal characters, this 
‘T'HECIDIUM so exactly resembles the recent species that a lengthened 
description would be only repeating what we have already written. 
