240 REPORT ON ALBATROSS MOLLUSCA DALL. 



common to minor groups in each of the three orders. In each (we are 

 coining to regard it as inevitable) certain members show affiliations with 

 members of the other orders. In each there are certain groups which 

 represent a relatively modern specialization carried so far as to be quite 

 peculiar. 



Pearliuess or a truly nacreous character of shell-substance is a source 

 of weakness. This kind of shell is more fully permeated with animal 

 matter, is more liable to decay and exfoliation, and is more readily 

 drilled by enemies than the aragonitic type of shell-substance which 

 conchologists call porcelhmous. The external prismatic layer which 

 usually accompanies a pearly interior layer, as iu Nucula, Trigonia, 

 Unio, etc., is very easily disintegrated. The tendency of evolution is to 

 promote the porcelhmous type. The older groups (Prionodesmacea and 

 Anomalodesmacea) contain all the pearly Pelecypods. Among the 

 Teleodesmacea there is not a single one. Furthermore, iu the two 

 former orders the most specialized and, developmental^/, the most mod- 

 ern forms are preferably porcellauous; those which we may reasonably 

 regard as of more ancient type tend to pearliuess. For example, in the 

 Anomalodesmacea the most striking instances of specialization are the 

 Pholads, Tubicolce and certain Myacea, all are earthy, or at least not 

 pearly. The Anatinaeea, which paleontologically are very ancient, are 

 largely pearly. The Prionodesmacea have few porcellauous groups, but 

 those which show this character, such as Ostrea and Pecten, generally 

 stand at the nearer end of a long line of progressive modification. 

 There are exceptions to this, such as Tindaria, in the Nueulacea, which 

 is obtrusively porcellauous. Leda and Solenomya, which retain so many 

 archaic features in their soft parts, have almost lost the pearly layer 

 while still falling short of the porcellauous character conspicuous in 

 most of the Teleodesmacea. The Areas, conspicuously earthy in their 

 shells, are modern in their total characters compared with the pearly 

 Nuculas. Turning to Gastropoda for a moment, we find that Pleuroto- 

 maria, one of the very earliest types of that class which can be recog- 

 nized 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 re- 

 sulted 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 sug- 

 gestions may be ventured upon. 



The radiating or transverse corrugations which we see iu ribbed 

 shells are not merely ornamental. They serve to add strength, while 

 they do not increase the weight, as would a corresponding thickening 

 of the shell. A familiar example of the same principle is afforded by 

 the corrugated sheet metal so frequently used by builders. Tbe ends 

 of these ribs impinge on the margin of the shell and crenulate it when 

 the shell is thin. Crenclla is a notable example. Many Mytilacea ex- 



