-±54 DalL — Hinge of Pelecypods and its Development. 



we may reasonably regard as of more ancient type tend to 

 pearliness. For example in tlie Anomalodesmacea the most 

 striking instances of specialization are the Pholads, Tubicolm 

 and certain Myacea, all are earth}" or at least not pearly. The 

 Anatinacea which paleontologically are very ancient, are 

 largely pearly. The P rionodesmaeea have few porcellanous 

 groups, but those which show this character, such as Ostrea and 

 Pecten, general^ stand at the nearer end of the long line of 

 progressive modification. There are exceptions to this, such as 

 Tindaria in the JVuctdacea which is obtrusively porcellanous, 

 while Leda and /Solenomya, which retain so many archaic fea- 

 tures in their soft parts, have almost lost the pearly layer while 

 still falling short of the porcellanous character conspicuous in 

 most of the Tehodesmacea. The Areas conspicuously earthy 

 in their shells are modern in their total characters compared 

 with the pearly !Nuculas. Turning to Gastropoda for a mo- 

 ment we find that Pleurotomaria, one of the very earliest types 

 of that class which can be recognized in the now existing fauna 

 is extremely pearly. On the whole the relation between the 

 two types of shell substance if not constant enough to be 

 called a rule is sufficiently so to be extremely suggestive. 



I have already suggested the mechanism of the infolding 

 which resulted in the cartilage and its supporting socket. It 

 is a very difficult task to account for the initiation of all the 

 types of teeth. A few suggestions may be ventured upon. 



The radiating or transverse corrugations which we see in 

 ribbed shells are not merely ornamental. They serve to add 

 strength while they do not increase the weight as would a cor- 

 responding thickening of the shell. A familiar example of 

 the same principle is afforded by the corrugated sheet metal so 

 frequently used by builders. The ends of these ribs impinge 

 on the margin of the shell and crenulate it when the shell is 

 thin. Crenella is a notable example. Many Mytilacea ex- 

 hibit a similar structure. These crenulations of the hinge line 

 and margin are not to be distinguished from nascent teeth and 

 have frequently been described as such by naturalists. JVucti- 

 locardia of Orbigny is a well known instance. The crenula- 

 tions of the margin are useful in securing a close tit between 

 the closed valves, whether at the cardinal or the basal margin. 

 But they would be more useful at the cardinal margin because 

 there they would prevent sliding of the valves upon one an- 

 other before they were completely closed, as do the long teeth 

 of the Nucxdaeea. Hence, it is probable that they would be 

 perpetuated and specialized there even if the ribbing disap- 

 peared from the exterior of the valves. Greater stress arising 

 from friction and pressure resisted, would tend towards the 

 thickening, widening and even buttressing of the cardinal 



