INSTITl'TK OK SCIENCE. PHlI.ADKI.l'HI A. 489 



The kind of teeth initiated at first would depend upon the sculpture and 

 form of the shell modified. It is obvious that with a shell having elevated 

 beaks distant from the hinge-line, and numerous ribs, many of the latter would 

 intersect the hinge-line nearly at right angles, so as to form a series of denticu- 

 lations comparable to those of Crenella, and constituting the prototype of such 

 teeth as were called Taxodont by Neumayr, included in the Prionodesmacea 

 by the writer. If the area between the beaks was smooth or only feebly stri- 

 ated, and ribbing began at the anterior and posterior angles of the umbonal 

 slope, these would necessarily intersect the hinge-line at an acute angle, or 

 even be nearly parallel to it, and plications induced by such ribbing would 

 tend to form in a direction parallel or nearly so to the general line of the hinge 

 (horizontally, instead of vertically), and would appear at the ends instead of 

 the center of the hinge-line. Such teeth are the laterals of Cyrtodonta, Tellina, 

 and the so-called laterals of Unio. If, as in the early Paleoconchs, the hinge 

 is symmetrically disposed with relation to the vertical from the beaks, the 

 laterals thus initiated will be equal and similar ; if the hinge-line is shorter 

 before the beaks the laterals at that end will necessarily be shorter also and 

 less horizontal. If the distance between the beaks is short, thus diminishing 

 the cardinal area over the hinge-line, it will also tend toward producing more 

 oblique crenulations of the hinge than in a species with a wide cardinal area. 

 These results follow necessarily from the most elementary principles of 

 mechanics, as any one may determine for themselves by drawing a circle with 

 numerous radii, extending from the center to the circumference, and cutting 

 off segments parallel to any diameter. The center will represent the beak ; 

 the radii, the ribs ; the chord of the segment cut off, the hinge-line. Do the 

 same with a cone, of which the apex is the center of radiation, and, finally, 

 give the cone a spiral twist ; in the last case the example will come very near 

 the conditions of the bivalve shell, and the sections made will clearly illustrate 

 the principles involved. 



Of course it need hardly be said to any intelligent biologist that these 

 conditions will produce, not the hinge of any bivalve exactly as we know it, 

 but modifications of the primitive hinge margin, which will be useful from the 

 moment of inception, which natural selection would at once lay hold of, and 

 from development and modification of which all existing types of hinges can 

 be legitimately derived. 



The general form of the pelecypod depends upon its principal anatomical 

 characters ; the size, number and position of the muscles, the presence, size, 

 and character of siphons, byssus, etc. Consequently to a certain limited ex- 

 tent, especially in the modification of the primitive simple Paleoconchs, the 

 differences of form would march with the respective anatomical differences as, 

 for example, those which retained the simply open mantle and subequal 

 adductors would continue to be of a rounded and symmetrical shape; while 

 those which tended to produce elongate siphons, or in which marked inequality 



