Dull — Hinge of Peleeypods and its Development. 451 



In the thin-shelled. Ouspidariidce a special buttress is often 

 developed to support the shell at this weak point. In the 

 Isocardiidce an independent cartilage was possibly never devel- 

 oped, but the infolding of the anterior part of the ligament 

 went far enough to leave permanent traces on the shell. That 

 it did not result in a cartilage if this was the case may possibly 

 be due to the fact that, owing to the great size and spiral char- 

 acter of the umbones, the anterior part of the ligament was 

 turned up instead of downward, and therefore did not tend to 

 shift toward the interior. 



If it is not clear how the thickening or vertical extension of 

 the ligament below the cardinal axis should cause its separation 

 into two parts, I need only recall the familiar experience of 

 every one in breaking off a wire or piece of tin by bending it 

 backward and forward on the line of the desired fracture. 

 The mechanical principles and results in the two cases are pre- 

 cisely similar. 



When finally developed in the same individual the ligament 

 and cartilage work in identically the same manner but in dif- 

 ferent directions The resistance of the ligament to compres- 

 sion prevents any straining of the adductors by a too wide 

 opening of the valves. The same resistance in the cartilage 

 prevents the ventral margins from crushing each other by sud- 

 den and, violent contractions of the adductors when the animal 

 is alarmed, and closes its valves. 



The nymphse, or processes to which the ligament is at- 

 tached, and. the fossette, or socket of the cartilage, have been 

 strengthened and regulated by the development of various but- 

 tresses and other devices, varying in different groups. The 

 cartilage in turn has its rigidity and strength increased in 

 many species by the special development of shell substance 

 known as the ossiculum. 



To return to the development of the cardinal margin. The 

 asymmetry of the shell and ligament relative to a vertical 

 transverse plane passing through the umbones, would be pro- 

 moted not only by the natural discrepancies between the ante- 

 rior and posterior halves of the body, but by the mechanical 

 effect of the projecting umbones. Where a shell opens later- 

 ally, in the strict sense of the word, unless the beaks are very 

 inconspicuous, or are separated by a wide projection of the car- 

 dinal border (as in Area noce), they will strike against and wear 

 out one another. This abnormal or accidental result is very 

 constantly observable in many Anati?iidce, such as our own 

 Thracia Conradi. But it must be a source of weakness and 

 danger to the animal. If the ligament is shifted posteriorly 

 the valves must open more obliquely, with a result that this 

 dangerous friction will be avoided in most cases. 



