1847. ] SHARPE ON THE GENUS TREMATIS. 67 
as inthe Orbicule ; this slit reaches from about the centre of the shell 
to near the posterior margin. Both the valves are slightly and irre- 
gularly convex, giving a depressed form to the shell. 
The valves are united by a hinge, of which the details cannot be 
seen in the specimens; but it is probably formed of two diverging 
lamellar processes in the dorsal valve, for where the shell of that 
valve has been worn away, we can trace three calcareous plates di- 
verging from the hinge of the dorsal valve, as in the Leptznoid spe- 
cies of Orthis and in many of the Spirifers. Wherever these plates 
are found in the Brachiopoda, the outer pair appear to be continua- 
tions of the teeth or lamellar processes of the hinge; so that the 
presence of such plates is sufficient to show that the valves played 
upon a hinge. The third or mesial plate separates the two great ad- 
ductor muscles. 
The beak of the dorsal valve is slightly produced, but the speci- 
mens are too imperfect to show whether there was any opening in it 
for the passage of a ligament. As the shell was attached by a ligament 
passing through the ventral valve, it is not probable that it should 
have had a second mode of attachment at the hinge, for we know no 
Brachiopod which has two modes of fixing itself. 
The shell consists of layers of two distinct structures: the outer 
layer is punctated ; the punctations are so large as to be clearly visible 
to the naked eye, and are arranged quincuncially with great regula- 
rity. The inner layers of shell are not punctated, and have a fibrous 
and slightly striated appearance and pearly lustre; these impunctate 
layers are thickest towards the middle of the shell and do not quite 
reach the margin*. ey: 
It thus appears that the genus Trematis differs from Orbicula in 
the punctated structure of its shell and in having the valves united 
by a hinge, while it is distinguished from Terebratula and the other 
hinged forms of Brachiopods by the ligament passing through the 
ventral valve. It thus forms a connecting link of great interest be- 
* An inner layer of unpunctated shell lining an outer punctated layer is 
of common occurrence among the Brachiopoda; it may be especially observed in 
the flat species of Orthis and in many Leptene, covering all the central parts of 
the shell and leaving the punctations open only round the margin. Among the° 
Producte and some species of Chonetes, the punctations are closed up everywhere, 
except at the edge, by a gradual deposit of shelly matter in the interior. In Crania, 
Thecidea, and some recent Terebratule, the punctations can only be seen in the 
interior round the edge of the shell; but in the majority of the recent species of 
Terebratula, they are equally visible over the whole shell. 
Taking a general view of these punctated shells, it appears that in a very large 
majority of cases the punctations only remain open round the margins of the 
valves, so that it is probable that whatever function was served by these minute 
perforations, its operation was confined to the margin of the shell. As the respira- 
tory process is carried on in this family by organs placed round the edges of the 
mantle, it seems probable that the punctations must be connected with that 
rocess. 
. The present communication shows the necessity of attending to the distinction 
between the punctated and nonpunctated forms of Brachiopoda, of which Mr. 
Morris has pointed out the importance in the Terebratule. It was the study of his 
paper on that subject that led me to observe the characters of the genus Trematis, 
and I am also indebted to Mr. Morris for further assistance in working them out, 
F2 
