20 Davidson—Recent and Tertiary Species of Thecidium. 
the description we have given of the interior of Thecidium Mediter- 
raneum will suit Thecidium Adamsi equally well. In none of the 
examples I have been able to examine could I find what Mr. Mac- 
donald describes and figures as the ‘ outer simple loop,’ following the 
contour of the shell, but lying a little within the granulated margin 
(see Pl. I., fig. 14, e), these ‘intermediate half-loops or hook-like 
processes (fig. 14, f) should not be represented as ‘free,’ for they 
adhere to the shell by their inner margin, as we have described to be 
the case in the recent species. Neither is the central ascending 
tongue-shaped process, nor the descending process, correctly drawn 
in the woodcuts illustrating Mr. Macdonald’s description. I am at 
a loss to understand Mr. Macdonald’s figure (op. cit., p. 518, fig. 2), 
for it would seem to represent at the same time the interior of the 
dorsal and ventral valves, which in such a position would be 
a material impossibility. 
Dr. Adams has informed me that this little shell is abundant in 
the lower part of the Calcareous Sandstone and in the upper portion 
of the Lower Limestone of Malta, and is usually associated with 
Orbitoides, Cidaris, Echinus, Scutella, and other forms likewise cha- 
racteristic of the situation, and never found in any of the overlying 
beds. 
From the above it would appear that three of the four Ter- 
tiary so-termed species are constructed exactly on the same 
model; and of these we may look upon Thecidium Mediterra- 
neum as the type, although the species appears to have first 
made its appearance in the Lower Oligocene period. Their 
structure is very different in detail from what we find in such 
shells as Th. vermiculare, Th. digitatum, Th. hieroglyphicum, 
Th. papillatum, .Th. Mayale, &c., from the Cretaceous and 
Jurassic periods, in which forms the ascending and descending- 
apparatus are much more complicated. The arrangement seen 
in the Mediterranean type did not, however, originate in the 
Tertiary period, for we find in the Cretaceous formation one or 
two forms, such as 7h. affine (Bosquet), which partake of the 
same character. The second Tertiary type (Zh. Barrett?) we 
have already shown to be also recent. 
EXPLANATION OF THE PLATES. 
Puate I, 
Fie. 1. Thecidium Mediterraneum, Risso, recent; Mediterranean ; a, b, en- 
larged. 
apd be Metitercneam: Interior of the dorsal valve enlarged. A, c, E, F, + 
and G, ‘ascending process;’ H, bridge-shaped process; 7, h, 
half-crescent or descending process; M, cardinal process; w, 
scar formed by the adjustors or ‘muscles adducteurs lateraux ou 
externes’ of M. Duthiers. 
3. T. Mediterraneum. Interior of the ventral valve, enlarged, and a 
little turned up so as to show under the cavity of the beak; 
5, small concave plates (‘coque’ of M. Lacaze Duthiers) to 
