Davidson— Recent and Tertiary Species of Thecidium. 17 
certainly presents a canal similar to that which exists in the other 
animals of the same group, and that this 
longitudinal canal is present in all their 
length, being almost confounded with 
the mantle, or with the margin of the 
body—for this last is located in the 
insertion of the arms ; that again these 
‘arms’ insert themselves by their basis, 
not on the intermediate or internal 
lamella or ‘descending crescent shaped 
processes, but on the edge of the 
‘ascending apparatus.’ ‘Their direc- 
tion is that of the lamina on which Interior of the Dorsal valve of Theci- 
they are supported; and they arrive, dium Mediterraneum, with the animal ; 
after having described the inflections ™@e™*e* 
already indicated, at the median point of the tongue-shaped pro- 
cess (Pl. I., fig. 2, @), where we can see their two extremities 
located. This arrangement is shown in the annexed woodcut, which 
is the one published by Mr. 8. P. Woodward and myself in the 
‘Annals of Nat. Hist.’ for May, 1852. The cirri of the ‘arms’ are 
long and flexible. 
“We need not, for the purpose we have in view, follow M. Duthiers 
much further in his observations in connection with the anatomy of 
the animal of Thecidium, but conclude by stating that the French 
zoologist believes, together with some other naturalists, that the 
respiration was probably effected partly by the ‘arms,’ which bear a 
great analogy to the gills of other Mollusca; and that he has not 
been able to discover any anal aperture—a fact strongly urged by 
Messrs. Huxley, Hancock, and Gratiolet, for Terebratula and 
Rhynchonella. 
M. Duthiers is of opinion that the sexes are distinct, or in other 
words that there is amale and female animal; but Mr. Hancock still 
considers this to be uncertain, and is disposed to the opinion that 
the sexes are combined in the same animal. 
Thecidium Mediterraneum occurs in large numbers in many parts 
of the Mediterranean from towards the extremity of the Gulf of 
Bone to near Cape Rosa, &c., living attached to corals and other 
marine objects, and ranging in a depth of between 40 and 80 fathoms. 
Mr. S. P. Woodward has detected the same shell in Sir Charles 
Lyell’s collection of Miocene fossils from the Grand Canary Island ; 
and lastly, Mr. Barrett procured two living examples of Thecidium 
Mediterraneum at 60 and 150 fathoms near Jamaica, and of one 
of his specimens we give a figure (PI. IL, fig. 5). 
2. THEecIpIum Barretti, Woodward, MS. Pl. IL, figs. 1, 2, 3. 
—Shell small, somewhat pyriformly ovate, attached to marine ob- 
jects by a portion of the back of its beak. Dorsal valve semicircular, 
flattened, and slightly convex at the umbone. Ventral valve some- 
what pyriform, very convex, deep, and thickened ; beak moderately 
produced ; area triangular, but more or less irregular on account of 
the position and extent of its attached surface. 
VOL. I.—NO. I. Cc 
