MR. R. OWEN ON THE ANATOMY OF THE BRACHIOPODA. 153 
On the Anatomy of Orbicula. 
I have not been able to find any account of the anatomy of this Brachiopod, beyond 
the statement of its possessing the spiral arms peculiar to the order. Cuvier, indeed, 
refers the soft parts to Poli’s genus Criopus!; but the animal so denominated belongs, 
as Mr. Sowerby observes®, to aspecies of Crania (Cran. personata, Sow.), a genus, how- 
ever, which, in its internal organization, is without doubt closely allied to Orbicula. 
Four recent and well preserved specimens of the species which Mr. Broderip in the 
preceding Memoir has termed Orb. lamellosa, were submitted to me for examination. 
The margin of the shell is of a soft texture and thickened, and the edges of all the 
layers of increase are more horny than calcareous. The layers of increase are large 
in proportion to the size of the shell, and are very irregular in their contour; the 
inside of the shell is smooth and polished. The flattened valve is perforated by a 
longitudinal fissure, measuring nearly three lines in length, and about half a line in 
width, and situated in the middle of an oval depression. Through this fissure the organ 
of adhesion, or the foot, passes, and immediately expands into a round sucker or disk, 
which fills up the whole of the depression, and conceals the margins of the slit. Im- 
mediately anterior to the fissure a longitudinal plate, about a line in length, projects 
into the interior of the shell for the extent of half a line; beyond this a broader ele- 
vated ridge is continued to within two lines of the anterior margin of the valve*. 
Along the whole circumference of the valves shining cilia are seen projecting for an 
extent varying from two to four lines. These arise all round the margins of both 
lobes of the mantle; they are much longer than in Terebratula and Lingula anatina, 
and are rather longer than in Ling. Audebardii, Brod., a new species discovered by 
Mr. Cuming. 
On carefully removing the imperforate valve, the vascular mantle is seen with the 
margin entire in the whole of its circumference. The muscles and viscera form a rounded 
mass, situated in the posterior half of the shell. First are seen the extremities of two 
muscles‘, of an oblong figure, converging anteriorly, and measuring two lines by nearly 
one: in the triangular space between these muscles is situated the green liver®, behind 
which is the grey ovary®; and at the posterior part of the circle are the extremities of 
two smaller muscles’. The four impressions of these muscles are observable on the 
interior of the shelly valve. 
On removing the lower valve, which should be cut through from either side as far as 
1 Testacea utriusque Siciliz, vol. ii. pl. xxx. Figg. 21-24. 2 Linn. Trans., vol. xiii. p. 471. 
’ This I regard as a rudimentary form of the internal calcareous apparatus of Terebratula; it represents the 
central process of support (c. Fig. 4. Plate xxi.). The mantle-lobe with two vessels, and the position of the 
alimentary canal, prove that the flattened valve of Orbicula, although perforated for the organ of adhesion, is 
really analogous to the imperforate valve of the Terebratule. 
4 f. f. Figg. 5. 7. 8. Plate xxi. 5 v. Figg. 5.11. 6 w. Figg. 5,11. 7g. g. Figg. 5.7. 8. 
