WOODWARD — SOME ACCOUNT OF EAURETTIA. 



377 



(Petref. vol. ii. pi. 1G5, fig. 5, c). AVe linvc ])rA'ovG pointed out that it 

 is essefitial there should be space for th(! alimentary canal to pass 

 between the hinge and posterior shell-niiisch! of a bivalve, and Ave 

 have shown that such an opening is provided in the ITip])uritc and 

 Eadiolite by the undercutting of the muscular apopliy-is, which 

 would otherwise close the whole interval (fig. 2, i). Tlierc 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 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 

 Caprotince, found in soft yellow marls of Le Mans, in the Department 

 of Sarthe, whicli may be cleared from the matrix without difficulty. 



Witli respect to the other suggestion, that the two depressions in 

 the lid of the Hippurite, (tlie ociiJi hiTL hioculaliis,) 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 orifices, 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 ' G-eological 

 Journal' (pi. 3, fig. 4), portions of the columns remain adhering to the 

 spots; aud 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 brokon 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. 



Barreitia moirUifera ; gronp of three iiulividuals, mucli reduced. 

 Longiltidinal section of the u|i])cr pai't of a hn\ae specimen, reduced one-fifth. 

 Longitudinal section of a fraginent, taken upon the line of uuiou of three iu- 



Trausvcrse section of the same specimen. 



Plate XXL 



Fig. 5. — Transverse section of the same s])ecimcn as Fig. 2 : z'/, line of section; 

 r, body-cavity of lower valve; v, unil)onal (-ivily: /, denial ])roccss of lower valve; t (' , 

 dental process of upper valve; a a', a{l(liict()r jirocesscs ; c c', carlilagc pits; m, muscular 

 inflection ; ?i, siphoual intlection ; i, probable course of alimentary canal and cxhaleut cur- 

 rent ; .r, canals and canaliculi of upper valve. 



Fig. C— Literior of up|)or valve of hippurite from Angoulemc, with part of the wall 

 of the lower valve adhering to it (marked .r) ; the inner shell-layer wanting. 



Fig. 7. — Interior of a lower valve from the same place. 



Fig.l.- 

 Fig. 2.- 

 Fig. 3.- 

 dividuals. 

 ¥vj^. 4.- 



TOL. T. 



