"-r-r- -, n.lmlfcnfrufguilj I IAM "' Ilinv !-, 



outline and .,!,-, M ti,-ntl\ of the size of the specimen. The rostral region of the 

 right valve was greatly reduced and flatten* -I. tint of the left not so much reduced 

 ami nia.lt> more strongly convex at .. In the lower half of the shell the result of 

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

 vahes to have been equal, we draw a curve through the point a that is intermedi.it. 

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

 tli.- -.happened one of the left. An equivalent curve is then drawn for the right 

 valve ami then continued to and beyond the point k We now have the original 

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

 other \.tl\o being equally curved, only in an opposite direction, is then easily 

 finished. 



Has MIL' s.,tisiied ourselves as to the original shape of the shell, it is first desirable 

 to determine whether the valves are equal, as in Modiolopsi* and Wlntrlln, or unequal, 

 ;i> in rttrinni ami Arislerclla. Next we note the relation of the various part* 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 l-eaks and umbones is taken into account. The former may be terminal 



-ituated 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 (Cuneamya, \Vhittlla), or they may 

 be U>s> full ami comparatively erect (Clionychia), or depressed, or scarcely distinguish- 

 al.le (< >/ !,,,!' It't and Clidophorus). Decided deviations in the position ami altitude 

 of the beaks are generally of generic value, but lesser modi Heat ions are likely to 

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

 prol .ably 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 alwence of a .byssal opening, of the 

 lii'uile ami 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 Inurement. The hinge may be edentulous, in which case an external ligament 

 (perhaps internal also) may usually be assumed if not found (see fig. 85, IV). In 

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



