TEREBRATULA. 
Puate XI. 
Species 47. (Fig. a, Mus. Taylor; Fig. 4, c, Mus. Cuming.) 
TEREBRATULA (MnGERLIA) TRUNCATA. Ter. testa trans- 
versim ovatd, plus minus semicirculari, fulvescente- 
aibd, ferrugineo-tinctd, liris dichotomis undique con- 
fertim radiatd, rostro brevissimé truncato, foramine 
peramplo, arei angustd, quasi sulcatd, deltidio vix 
nullo; valvis subequalibus, modicé convewis, dorsali 
medio late indentatd, ventrali tumidiusculd ; apophyse 
brevi, dupliciter affivd, septo centrali bilobato, margini- 
bus intus spinulosis. 
THE TRUNCATED Mrceriia. Shell transversely ovate, 
more or less semicircular, fulvous-white, stained with 
rust, closely rayed throughout with dichotomous 
ridges, beak very shortly truncated, foramen very 
large, area narrow, as though grooved, deltidium 
rudimentary ; valves nearly equal, moderately convex, 
the dorsal broadly indented in the middle, ventral 
moderately swollen; apophysis short, doubly attached, 
central septum bilobed. 
Anomia truncata, Linneeus, Syst. Nat. p. 1152. 
Anomia disculus, Pallas. 
Terebratula truncata, Lamarck. 
Terebratula disculus, De Blainville. 
Terebratella truncata, D’Orbigny. 
Terebratula monstrosa, Scacchi. 
Orthis truncata, Philippi. 
Megerlia truncata, King. 
Orthis oblita, Michelotti. 
Megathiris oblita, D’Orbigny. 
Hab. Mediterranean (affixed to corals, at a depth of from 
fifty to a hundred fathoms). Cape Finisterre (at a 
depth of ninety fathoms). Canaries. 
The beak of this species is so shortly truncated, that 
the foramen of the shell is always more or less eroded, 
encroaching upon the umbo of the dorsal valve. Though 
comparatively unique in form, the apophysis does not very 
creatly differ from that of Terebratella. It is a shortly 
reflected loop, broadly doubly attached to a central septum, 
but the loop between the two points of attachment has 
an additional process or lobe on each side. Mr. Jeffreys 
states that he possesses a specimen collected by Dr. Turton 
in Torbay, but its British habitat has not been confirmed 
by subsequent researches. 
Species 48. (Fig. a, 6, Mus. Taylor; Fig. c, d, Mus. 
Cuming). 
TEREBRATULA (THECIDEA) Mrpirerranea. er. testd 
subpyriformi-ovatd, crassi, irregulari, sordide alba aut 
Sulvescente, rostro calloso, validé producto, trigono, im- 
perforato, area deltidioque peramplis, cardine utrinque 
calloso-dentato, valvd dorsali concavo-pland, ventrali 
crasse converd, radiatim striato-suleatd, margine intus 
serrato; apophyse calloso-affind, quadrilobatd. 
Tar MepirerraNgaNn TuecrpEa. Shell somewhat py- 
riformly ovate, thick, irregular, dead-white or ful- 
vous, beak callous, very much produced, triangular, 
imperforate, area and deltidium very large, hinge 
eallously toothed on each side, dorsal valve concavely 
flattened, ventral thickly convex, radiately striately 
grooved, margin serrated within, apophysis callously 
affixed, four-lobed. 
Risso, Eur. Mérid. pl. iv. f. 183. 
Thecidea testudinaria, Michelotti. 
Thecidea spondytea, Scacchi. 
Hab. Mediterranean (attached to corals). 
Thecidea is vather a difficult subject to observe, by rea- 
son of its habits. It bears much the same relation to the 
rest of the Zerebratule that Hinnites bears to Pecten. 
The pedicle, like the byssus in that genus, loses its func- 
tion, and the animal affixes itself by the shell. The shell, 
as in most such cases, is of extremely callous and irregular 
growth, and it is only by the.casual development of its 
structural details in,a numerous series of specimens, that 
its characters can be ascertained. In some fossil forms of 
Thecidea a minute terminal foramen has been shown to 
exist, but I find no trace of one in the recent species. The 
umbo of the ventral valve is largely produced into a thick- 
ened spondyloid beak, and in the front area of it, a large 
triangular rudimentary deltidium is soldered. The dorsal 
valve, rudely indented and flattened, is of a horse-shoe shape, 
and the ventral valve, densely convex, is rayed outwardly 
with close-set grooves, which produce serrations on the 
inner margin. The hinge is composed of distinct tooth- 
like processes in the ventral valve, interlocking with corre- 
sponding sockets in the dorsal; and the apophysary sys- 
tem is modified into a kind of fixed ridge, accompanied 
by a profusion of little indentations. When describing 
Thecidea twenty years ago in my ‘Conch. Systematica’ 
(vol. i. p. 184), I remarked that “the upper valve is flat 
| and curiously indented, as if to fit certain correspond- 
ing parts in the body of the animal. These indentations, 
which spread round in a semicircular direction from the 
_ hinge, look exactly as if they were picked out in wax; and 
in a specimen we have examined with considerable minute- 
ness, they were filled with the dried remains of numerous 
fine cilia.”’ No sort of loop had been observed in Thecidec, 
and it is now obvious that this bed of the ciliary arms is a 
modification of it. 
February, 1861. 
