LAMELLIBEANCHIATA. 483 



Preservation and methods ol study.! 



outline and consequently of the size of the specimen. The rostral region of the 

 right valve was greatly reduced and flattened, that of the left not so much reduced 

 and made more strongly convex at a. In the lower half of the shell the result of 

 distortion was reversed in the two valves. In the restoration, assuming the two 

 valves to have been equal, we draw a curve through the point a that is intermediate 

 in convexity between the flattened curve of the rostral half of the right valve and 

 the sharpened one of the left. An equivalent curve is then drawn for the right 

 valve and then continued' to and beyond the point h. We now have the original 

 outline of one of the valves as it would appear in an end view. The outline of the 

 other valve being equally curved, only in an opposite direction, is then easily 

 finished. 



Having satisfied ourselves as to the original shape of the shell, it is first desirable 

 to determine whether the valves are equal, as in Modiolopsis and Whitella, or unequal, 

 as in Pterinea and Aristerelta. Next we note the relation of the various parts of the 

 outline to each other, the relative width of the two ends and other features bearing 

 upon the determination of the contour. Now the position, altitude and degree of 

 fulness of the beaks and umbones is taken into account. The former may be terminal 

 (i. e., situated at the anterior extremity of the hinge line and projecting as far forward 

 as the margin beneath them), as in Ambonychia; or they may be nearly central in 

 position, as in some species of Ctenodonta. Then the umbones may be strongly 

 gibbous and the beaks curve over the hinge line {Cunedmya, Whitella), or they may 

 be less full and comparatively erect (Glionychia), or depressed, or scarcely distinguish- 

 able (Cycloconcha and Clidophorus). Decided deviations in the position and altitude 

 of the beaks are generally of generic value, but lesser modifications are likely to 

 prove of merely specific importance. The character of the surface markings will 

 probably have been taken into account at once. 



In the next order, and here we usually credit them with generic and greater 

 value, the student should observe the presence or absence of a byssal opening, of the 

 lunule and escutcheon, and the character of the area. He should note also whether 

 the edges of the valves fit tightly or gape at one or both ends or ventrally. His next 

 step is to observe the position, distinctness and relative size of the various muscular 

 impressions, the adductors particularly. Nor is he to forget to trace out the pallial 

 line. Next he may find internal sockets, plates or ridges, that supported internal 

 ligaments, or to which muscles were attached. Finally, he will observe the method 

 of hingement. The hinge may be edentulous, in which case an external ligament 

 (perhaps internal also) may usually be assumed if not found (see fig. 35, IV). In 

 Modiolopsis there may be a slight thickening or rudimentary cardinal tooth in each 



