10 



BRITISH FOSSIL TRIGONLE. 



closed in the following manner : — The hinge allowed of a slight vertical or sliding motion 

 to the valves by which the mollusk was enabled to bring the produced and internally 

 convex lip, forming the extremity of the sulcus in the left valve, opposite to and in con- 

 tiguity with the open extremity of the carina of the right valve, which it exactly 

 fitted ; in the same manner the sulcus of the right valve closed the extremity of the carina 

 of the left valve, as seen in our engraving. By this exertion of muscular power, therefore, 



the valves were firmly locked at the will of the mollusk. 

 Another result also followed this arrangement : the orifice for 

 the excurrent aperture existed at the border of the depressed 

 or superior half of the area ; and, as that of the right valve is 

 slightly lower than the corresponding portion of the other 

 valve, the border forms a narrow orifice or undulation which 

 became close-fitting when the incurrent aperture was also 

 closed. Specimens, as in T. elongata, frequently occur with the 

 apertures closed and locked ; in other instances the extremities 

 of the carinas are rendered open by the relaxation of muscular 

 force at the instant of death. In the Clavellatce and other 

 sections generally there were no open extremities of caringe, 

 and the excurrent and incurrent orifices were closed by simple muscular effort, 

 but as the respiratory orifices of the Costata were not closed by simply shutting 

 the valves, it became necessary to protect those organs by a special contrivance 

 which is no less remarkable for its simplicity than its efficiency ; it is, in fact, an exact 

 reproduction of the same design as exhibited in the more ancient and allied genus 

 Myophoria. From the foregoing statement it might be concluded that the Costata re- 

 present the most ancient or primordial portion of the genus Trigonia; but, judging from 

 the present state of our knowledge of the organic contents of the Liassic and Triassic 

 strata, such an inference is scarcely justifiable ; certain it is that other sectional forms of 

 Trigonise occur in Upper Liassic deposits, and that Ave are altogether unacquainted with 

 the genus in the middle and lower subdivisions of that great formation. 



In the left valve the extremities of costae terminate abruptly posteally, and are 

 separated from the carina by the ante-carinal sulcus. As there is no sulcus in that 

 position in the right valve, the costse touch the marginal carina, and sometimes pass over 

 it as so many plications. The aperture formed by the extremities of the two marginal 

 carinas does not form a lengthened canal internally as in the genus Myophoria, the depo- 

 sition of nacreous deposit went on simultaneously with the growth of the valves, so that 

 whatever may be the stage of growth, the figure of an inner canal is preserved even in 

 large species only for a length of about five or six lines. 



The cardinal processes are unusually large and massive. In T. sculpta they occupy 

 nearly a third part of the interior of the valves ; thus it follows that the valves at their 

 apical portions have always much convexity. 



