14. Davidson—Recent and Tertiary Species of Thecidium. 
are extremely variable in the different species composing the 
genus; in some they are very complicated, while in others 
they are comparatively simple; but it is not the object of the 
present communication to review the whole genus, or to give 
a description of its many and varied species, these details 
having been published in the admirably illustrated memoirs 
already named; we will therefore at once proceed with the 
_ description of the two recent species, so as to be able after- 
wards to establish a comparison between them and the Tertiary 
forms hitherto discovered. 
TuHrcipium MepiTrerRANEUM, Risso. Pl. L., figs. 1, 2, 3; and Pl. II., 
figs. 5 to 10. Thecidea Spondylea, Sacchi.—In external form 
this small shell is somewhat pyriformly ovate, very variable in shape, 
and in life it is attached to marine objects by a portion of the back 
of its beak. The dorsal or smaller valve is thin, semicircular, and 
slightly convex at the umbone, flattened near the margin ; the hinge- 
line is straight and shorter than the breadth of the shell, a small 
triangular hinge-area being likewise observable. ‘The ventral or 
larger valve is more or less regularly pyriform, very convex, 
thickened, and somewhat depressed longitudinally along the middle. 
The beak is much produced, callous, and, when well shaped, triangu- 
lar ; but more often somewhat irregular, on account of the position 
and extent of its attached surface. The area is large, triangular, 
and flat, with a slightly elevated, but flattened deltidium. Shell- 
structure punctate. The interior arrangements have been already 
described by several naturalists, but a glance at the accompanying 
illustrations will convey to the reader’s mind a much clearer impres- 
sion than words could effect. In the interior of the dorsal valve an 
oblong or squarish concave, prominent, cardinal process exists be- 
tween the dental sockets (Pl. I. fig. 2, a) ; and outside of each of the 
socket-depressions is seen an oval muscular scar (wW), which M. 
Duthiers attributes to his ‘lateral adductor muscles’ (‘adjustors’ of 
Hancock). <A broad, thickened, sloping, granulated margin encircles 
the valve, and forms a bridge () over the small, deep, visceral 
cavity, and close to the basis of the cardinal process. The granu- 
lations are larger and most prominent as they recede from the 
outer margin. This inner denticulated or granulated margin follows 
in a parallel manner the margin of the shell from the bridge-shaped 
process (H), until it reaches near to the middle of the front at c, 
where it suddenly stops to become inflected upwards. At the point 
c the inner margin is again directed upwards, producing a second 
parallel curve, when at E, by another downward curve, it forms a 
third short parallel concave curve, until reaching the point (F) near 
the centre of the valve, where it combines with the similar inflec- 
_tions of the other half of the shell, so as to produce, on the median 
line, an upwardly produced tongue-shaped process (G), the angular 
extremity of which is directed towards the middle of the bridge- 
shaped process (1). These four symmetrically bent ridges or lobes 
