No. 2.] FILAMENTS IN THE LAM ELLIBRANL 11 ('.ILL. 77 



detailed in an earlier paper, 1 1 consider the simplicity and smooth 

 surface of the gill to be secondary characters --a retrograde 

 development from the folded gill type of the Veneridae. Fusion 

 was also never 6bserved in the filamentous type of gill. 



The results of my own observations upon the folded lamellar 

 gills may be tabulated as follows : 



1. Fusion strongly developed: Cardium edulc Lin., Chania 

 pcllncida Brod., Batissa tenebrosa Hinds, Psammobia vespertina 

 Lin., Donax serra Chemn. 



2. Fusion moderately developed : Venus verrucosa Lin., 

 Cyprina islandica Lin., in latter strongly developed on transi- 

 tion line from outer gill to appendage. 



3. Fusion very slightly developed : E?isatc!laamcricanaVQrr\\\, 

 Mya arcnaria Lin. , Donax politus Poli, in latter perhaps accidental. 



4. No fusion observed : Cythcrea cJiionc Lin., Donax trun- 

 culus Lin., Ostrca virginiana Lister, TJiracia papyracea Poli. 



Lima and Ostrea should probably be added to the list of forms 

 in which fusion occurs, though on somewhat doubtful evidence, 

 which will be mentioned later. 



Thus we find this fusion of filaments similarly developed in 

 widely separated forms, belonging to very diverse groups ; on 

 the other hand, within the single genus Donax we find all 

 grades of fusion, as we also find all grades of folding. 



This parallelism of folding and fusion in the genus Donax 

 appears to me to furnish, in a certain sense, an epitome of the 

 whole matter, for I consider the fusion of the filaments to be 

 a mechanical correlative of the folding of the lamellae. The 

 process may be pictured in something this way. The folding of 

 the lamellae is gradually developed in the young lamellibranch 

 as the increasing number of filaments becomes too great to lie 

 in one plane. But at the free margin the filaments are bound 

 somewhat firmly together and the folding is somewhat reduced. 

 This leads to a crowding together of the filaments at this point 

 and eventually to an organic fusion of the same. How great 

 the crowding must really be in the strongly folded forms may 

 be inferred from the fact that the upper part of the inner gill of 



1(1 Die systematische Verwertbarkeit der Kiemen bei den Lamellibranchiaten," 

 Jenaische Zeitschrift fiir Naturivissenschaft. Bd. xxxi. 1897. 



