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hinge composed of a sharply hooked tooth-like process at each 

 corner of the deltidium in the upper valve, very closely fitting 

 a corresponding socket and process in the under valve ; no liga- 

 ment ; under valve developed into a shield at the umbo, which 

 passes into the upper valve at the axis of the hinge below the 

 deltidium, and has in connection with it, internally, an extended 

 apophysary system of variously looped horny or calcareous pro- 

 cesses. 



We include in genus Terebratula all the Brachiopods, fifty-four in num- 

 ber, having shells of the form of an inverted Roman lamp, minutely 

 punctured throughout. Three other Brachiopods (genus Bhynchonelld) 

 have similar lamp-shaped shells, but they are not punctured, and along 

 with this simple conchological difference, there are differences both of a 

 malacological and a geological character. Terebratidoa are found in all 

 parts of the world, (three very characteristic species, T. caput-serpentis, cra- 

 nium, and cistellula inhabiting our own seas) often at very great depths, 

 adhering to stones, shells, and corals, and many of them, especially from 

 the tropics, are beautifully coloured, often deep rose or scarlet variegated 

 with red stripes. The valves being dorsal and ventral, not lateral, with 

 respect to the soft parts, as in the Lamellibranchiate order of bivalves, the 

 animal seems to dwell in an inverted position; the smaller valve upon which 

 it rests is the dorsal, and the ventral valve projects over the hinge in the 

 form of a beak, perforated and serving for the passage of a tendinous 

 pedicle, by which the creature is moored to its place of attachment. The 

 valves of Terebratula are hinged without the intervention of any ligament. 

 Beneath the ledge of the beaked projection of the upper valve is a hooked 

 tooth, which fits closely into a corresponding socket and tooth-like pro- 

 cess in the lower valve, and the umbonal extremity of the lower valve 

 passes deeply into the beaked portion of the upper valve, and is dove-tailed 

 into it, so to speak, in many species, by a plate of one or more pieces, called 

 the deltidium. Inwardly the under valve forms a kind of shield, and in 

 connection with it is a variously elaborated system of apophyses, or looped 

 skeletons, which give support to the remarkable development of cirrhated 

 arms or brachia, after which the class is named. 



The modifications of the hinge and of the brachial apophyses of Tere- 

 bratula are rather numerous, and afford abundant characters for subdivi- 

 sion. In T. vitrea (Terebraltda proper) the loop is short and formed of 

 two ribbon-shaped lamellae proceeding from the shield or crural base ; in 

 T. caput-serpentis {Terebratulina, D'Orb.) the internal appendage is very 

 small, arched and vaulted below ; in T. dilatata ( Waldheimia, King) the 

 loops are long ; in T. Chilensis (Terebratella, D'Orb.) the loops are also 

 long, but the crura give off about their middle a flat, wide, horizontal pro- 



