74 _ MONOMYARIA. TEREBRATULA, 
generally trigonal and gibbous; attached by a short 
peduncle to extraneous marine bodies; the larger or upper 
valve, with a projecting umbo, frequently bent, and perfo- 
rated at its apex, or notched at its inner edge, and having 
a small curved tooth on each side of its hinge, which fits 
into a corresponding pit in the opposite valve; the inside 
of the smaller valve is provided with two slender testaceous 
processes, which are sometimes simple, short and recurved ; 
at others considerably elongated, branched, bent in various 
directions, and anastomosing for the most part; sometimes 
they are situate near the centre of the valve, and in other 
instances are united by their points to the shell; these 
usually emanate from each side of the hinge; both valves 
provided with two nearly obsolete muscular impressions, 
but sometimes they are strongly developed ; those of the 
larger or perforated valve are oblong, central, and close to 
each other ; in the smaller valve they are triangular, with 
their angles rounded, also nearly central, but more distant 
than in the other valve. 
Terebratula semiglobosa. Plate VII. figs. 17, 18; and 
T. inconstans. Plate VII. fig. 28. 
The Terebratule are extremely variable in their general shape, being some- 
times oviform, sometimes much depressed, more especially in their young 
state. The peduncle is formed of numerous closely united fibres, which are 
separated at their outer end, by which they are attached. The manner in 
which the teeth are fitted, renders it impossible to detach the valves, without 
either breaking them or the edges of their sockets. 
The shells of this genus are distinguished from the Anomiae, by their form 
being quite regular ; by their having a perforated produced beak in one valve, 
from whence emanates the fibrous pedicle by which they are attached, whereas 
the Anomiz are attached by means of a testaceous plug ; and they are divested 
of the internal appendages which are peculiar to the Terebratule; they can- 
not be mistaken for Lingule or Orbicule, as these shells have no hinge; and 
they will be distinguished from Hipponyx, Thecidea and Crania, because these 
genera are always attached to marine bodies by their flat valve. 
The Terebratule are Oceanic shells, adhering to extraneous substances. 
They are very numerous, and inhabit the seas of all countries; twe or three 
are natives of the British seas. They are subject to considerable variety in 
form and markings, 
Fossil species are still more numerous than the recent ones. They are of 
the utmost importance to geologists, as they occur in all the Secondary, and 
Tertiary strata, commencing with the earliest, excepting those of fresh water 
origin, and they have never been detected in the upper marine formation. 
Particular species distinguish certain strata. 
Section 1. Smooth, generally oblong; the middle of the front even or 
depressed. 3 
Section 2, Plaited, generally oblate; the middle of the front even or 
elevated, 
