WOODWAED — SOME ACCOUNT OF BAERETTIA. 
377 
(Petref. vol. ii. pi. 165, fig. 5, e). We have before pointed out that it 
is essential there should be space for the alimentarv canal to pass 
between the hinge and posterior shell-muscle of a bivalve, and we 
have shown that such an opening is provided in the Hippurite and 
Eadiolite by the undercutting of the muscular apophyi^is, which 
would otherwise close the whole interval (fig. 2, i). There is a 
hippurite in the British Museum which is hollow and empty, having 
been lined with only a thin film of spar. It is broken open at the 
side, and a wire has been passed round in the direction of the dotted 
line in fig. 5, i x, which is the course that must have been taken by 
the alimentary canal in the living animal. The nearest approxima- 
tion to the hinge-structure of this genus is presented by the little 
CaprotincB, found in soft yellow marls of Le Mans, in the Department 
of Sarthe, which may be cleared from the matrix without difiiculty. 
With respect to the other suggestion, that the two depressions in 
the lid of the Hippurite, (the oculi in H. hioculaius,) may be openings 
to facilitate the admission and escape of the branchial currents, it is 
only needful to observe that they have no existence as orijices, except 
in weathered specimens. These spots in the operculum correspond 
to the projecting columns in the lower valve, and fit down upon them 
closely. In the upper valve of H. Loftusi, figured in the ' Geological 
Journal' (pi. 3, fig. 4), portions of the columns remain adhering to the 
spots ; and in the specimens now represented from Angoulerae (figs. 
6 and 7) the removal of the inner layer of shell has exposed the cor- 
responding columns in each valve, while a portion broken from the 
lower valve is still attached to the upper, and shows the closeness of 
the contact at the place of the imaginary openings. The probable 
relation of the second column {n) to the respiratory currents of the 
animal was first suggested in our former descriptions. 
EXPLANATION OF THE PLATES. 
Plate XX. 
Fig. 1. — Barrettia monilifera ; group of three indivitluals, much reducecl. 
Fig. 2. — Longitudiual section of the upper part of a large specimen, reduced one-fifth. 
Fig. 3. — Longitudiual section of a fragment, taken upon the line of union of three in- 
dividuals. 
Fig. 4. — Transverse section of the same specimen. 
Plate XXI. 
Fig. 5. — Transverse section of the same specimen as Fig. 2 : Id, line of section; 
r, body-cavity of lower valve ; u, umbonal cavity ; /, dental process of lower valve ; t t', 
dental process of upper valve ; a a', adductor processes ; c c', cartilage pits ; m, muscular 
inflection ; n, siphoual inflection ; i, probable course of alimentary canal and exhalent cur- 
rent ; x, canals and canaliculi of upper valve. 
Fig. 6. — Interior of upper valve of hippurite from Angouleme, w ith part of the wall 
of the lower valve adhering to it (marked a-) ; the inner shell-layer wanting. 
Fig. 7. — Interior of a lower valve from the same place. 
YOL. Y. 
3c 
